diff --git a/src/quartermaster/main.py b/src/quartermaster/main.py
index 3c1a2ad..30df371 100644
--- a/src/quartermaster/main.py
+++ b/src/quartermaster/main.py
@@ -6,6 +6,7 @@ from fastapi import FastAPI
 from fastapi.staticfiles import StaticFiles
 
 from quartermaster.routes import router
+from quartermaster.routes_month import router as month_router
 
 STATIC_DIR = Path(__file__).parent / "static"
 
@@ -14,6 +15,7 @@ def create_app() -> FastAPI:
     app = FastAPI(title="Quartermaster", version="0.1.0")
     app.mount("/static", StaticFiles(directory=str(STATIC_DIR)), name="static")
     app.include_router(router)
+    app.include_router(month_router)
     return app
 
 
diff --git a/src/quartermaster/routes.py b/src/quartermaster/routes.py
index eb7c6c9..b208268 100644
--- a/src/quartermaster/routes.py
+++ b/src/quartermaster/routes.py
@@ -8,7 +8,7 @@ from fastapi.responses import HTMLResponse
 from fastapi.templating import Jinja2Templates
 from sqlalchemy.orm import Session
 
-from quartermaster import service
+from quartermaster import month_service, service
 from quartermaster.db import get_session
 from quartermaster.models import SECTION_LABELS, Section
 
@@ -62,6 +62,7 @@ def index(request: Request, db: Session = Depends(get_session)) -> HTMLResponse:
     sections = [_section_view(db, s) for s in Section]
     target = service.get_debt_target(db)
     debt_minimums = service.list_entries(db, Section.debt_minimum)
+    current_ym = month_service.current_year_month()
     return templates.TemplateResponse(
         request,
         "index.html",
@@ -69,6 +70,8 @@ def index(request: Request, db: Session = Depends(get_session)) -> HTMLResponse:
             "sections": sections,
             "target": target,
             "debt_minimums": debt_minimums,
+            "current_year_month": current_ym,
+            "all_months": month_service.list_months(db),
         },
     )
 
diff --git a/src/quartermaster/routes_month.py b/src/quartermaster/routes_month.py
new file mode 100644
index 0000000..9908829
--- /dev/null
+++ b/src/quartermaster/routes_month.py
@@ -0,0 +1,224 @@
+from __future__ import annotations
+
+from decimal import Decimal, InvalidOperation
+
+from fastapi import APIRouter, Depends, Form, HTTPException, Request
+from fastapi.responses import HTMLResponse, Response
+from sqlalchemy.orm import Session
+
+from quartermaster import month_service, service
+from quartermaster.db import get_session
+from quartermaster.models import SECTION_LABELS, Month, Section
+from quartermaster.routes import templates
+
+router = APIRouter(prefix="/month/{year_month}", tags=["month"])
+
+
+def _parse_amount(raw: str, *, allow_zero: bool = True) -> Decimal:
+    try:
+        amount = Decimal(raw.strip())
+    except (InvalidOperation, AttributeError) as exc:
+        raise HTTPException(status_code=400, detail="amount must be numeric") from exc
+    if amount < 0:
+        raise HTTPException(status_code=400, detail="amount must be non-negative")
+    if amount == 0 and not allow_zero:
+        raise HTTPException(status_code=400, detail="amount must be positive")
+    return amount.quantize(Decimal("0.01"))
+
+
+def _require_year_month(year_month: str) -> str:
+    if not month_service.valid_year_month(year_month):
+        raise HTTPException(
+            status_code=404, detail="year_month must be formatted as YYYY-MM"
+        )
+    return year_month
+
+
+def _require_month(db: Session, year_month: str) -> Month:
+    _require_year_month(year_month)
+    month = month_service.get_month(db, year_month)
+    if month is None:
+        raise HTTPException(status_code=404, detail="month has not been created yet")
+    return month
+
+
+def _section_views(month: Month) -> list[month_service.MonthSectionView]:
+    return [
+        month_service.section_view(month, s, SECTION_LABELS[s]) for s in Section
+    ]
+
+
+def _render_section(
+    request: Request, month: Month, section: Section
+) -> HTMLResponse:
+    view = month_service.section_view(month, section, SECTION_LABELS[section])
+    return templates.TemplateResponse(
+        request,
+        "partials/month_section.html",
+        {"month": month, "section": view},
+    )
+
+
+def _render_target(request: Request, db: Session, month: Month) -> HTMLResponse:
+    target = month_service.get_month_target(db, month)
+    debt_minimums = [e for e in month.entries if e.section == Section.debt_minimum]
+    debt_minimums.sort(key=lambda e: e.id)
+    return templates.TemplateResponse(
+        request,
+        "partials/month_target.html",
+        {"month": month, "target": target, "debt_minimums": debt_minimums},
+    )
+
+
+@router.get("", response_class=HTMLResponse)
+def view_month(
+    year_month: str,
+    request: Request,
+    db: Session = Depends(get_session),
+) -> HTMLResponse:
+    _require_year_month(year_month)
+    month = month_service.get_month(db, year_month)
+    prev_ym = month_service.shift_year_month(year_month, -1)
+    next_ym = month_service.shift_year_month(year_month, 1)
+    all_months = month_service.list_months(db)
+    if month is None:
+        return templates.TemplateResponse(
+            request,
+            "month_create.html",
+            {
+                "year_month": year_month,
+                "prev_year_month": prev_ym,
+                "next_year_month": next_ym,
+                "all_months": all_months,
+            },
+        )
+    return templates.TemplateResponse(
+        request,
+        "month.html",
+        {
+            "month": month,
+            "year_month": year_month,
+            "prev_year_month": prev_ym,
+            "next_year_month": next_ym,
+            "all_months": all_months,
+            "sections": _section_views(month),
+            "target": month_service.get_month_target(db, month),
+            "debt_minimums": sorted(
+                (e for e in month.entries if e.section == Section.debt_minimum),
+                key=lambda e: e.id,
+            ),
+        },
+    )
+
+
+@router.post("/create")
+def create_month(
+    year_month: str,
+    db: Session = Depends(get_session),
+) -> Response:
+    _require_year_month(year_month)
+    try:
+        month_service.create_month(db, year_month)
+    except ValueError as exc:
+        raise HTTPException(status_code=400, detail=str(exc)) from exc
+    return Response(
+        status_code=204,
+        headers={"HX-Redirect": f"/month/{year_month}"},
+    )
+
+
+@router.post("/sections/{section}/entries", response_class=HTMLResponse)
+def add_month_entry(
+    year_month: str,
+    section: Section,
+    request: Request,
+    name: str = Form(...),
+    planned: str = Form(...),
+    db: Session = Depends(get_session),
+) -> HTMLResponse:
+    month = _require_month(db, year_month)
+    clean_name = name.strip()
+    if not clean_name:
+        raise HTTPException(status_code=400, detail="name is required")
+    month_service.add_month_entry(
+        db, month, section, clean_name, _parse_amount(planned)
+    )
+    db.refresh(month)
+    return _render_section(request, month, section)
+
+
+@router.delete("/entries/{entry_id}", response_class=HTMLResponse)
+def delete_month_entry(
+    year_month: str,
+    entry_id: int,
+    request: Request,
+    db: Session = Depends(get_session),
+) -> HTMLResponse:
+    month = _require_month(db, year_month)
+    section = month_service.delete_month_entry(db, month, entry_id)
+    if section is None:
+        raise HTTPException(status_code=404, detail="entry not found")
+    db.refresh(month)
+    response = _render_section(request, month, section)
+    if section == Section.debt_minimum:
+        target_html = _render_target(request, db, month).body.decode()
+        response = HTMLResponse(response.body.decode() + target_html)
+    return response
+
+
+@router.post("/entries/{entry_id}", response_class=HTMLResponse)
+def update_month_entry(
+    year_month: str,
+    entry_id: int,
+    request: Request,
+    name: str | None = Form(None),
+    planned: str | None = Form(None),
+    applied: str | None = Form(None),
+    db: Session = Depends(get_session),
+) -> HTMLResponse:
+    month = _require_month(db, year_month)
+    clean_name: str | None = None
+    if name is not None:
+        clean_name = name.strip()
+        if not clean_name:
+            raise HTTPException(status_code=400, detail="name must not be empty")
+    parsed_planned = _parse_amount(planned) if planned is not None else None
+    parsed_applied = _parse_amount(applied) if applied is not None else None
+    updated = month_service.update_month_entry(
+        db,
+        month,
+        entry_id,
+        name=clean_name,
+        planned=parsed_planned,
+        applied=parsed_applied,
+    )
+    if updated is None:
+        raise HTTPException(status_code=404, detail="entry not found")
+    db.refresh(month)
+    return _render_section(request, month, updated.section)
+
+
+@router.post("/target", response_class=HTMLResponse)
+def update_month_target(
+    year_month: str,
+    request: Request,
+    month_entry_id: str = Form(""),
+    db: Session = Depends(get_session),
+) -> HTMLResponse:
+    month = _require_month(db, year_month)
+    raw = month_entry_id.strip()
+    target_id: int | None
+    if raw == "":
+        target_id = None
+    else:
+        try:
+            target_id = int(raw)
+        except ValueError as exc:
+            raise HTTPException(
+                status_code=400, detail="month_entry_id must be an integer"
+            ) from exc
+    try:
+        month_service.set_month_target(db, month, target_id)
+    except ValueError as exc:
+        raise HTTPException(status_code=400, detail=str(exc)) from exc
+    return _render_target(request, db, month)
diff --git a/src/quartermaster/static/app.css b/src/quartermaster/static/app.css
index 13c3aa8..167b549 100644
--- a/src/quartermaster/static/app.css
+++ b/src/quartermaster/static/app.css
@@ -156,3 +156,124 @@ button[type=submit] {
 .target-section .section-header {
   border-bottom-style: dashed;
 }
+
+/* Monthly view ----------------------------------------------------------- */
+
+.month-nav {
+  display: flex;
+  align-items: center;
+  gap: 0.75rem;
+  padding: 0.5rem 0;
+  margin-top: 1rem;
+  flex-wrap: wrap;
+}
+
+.month-nav .month-label {
+  font-size: 1.2rem;
+  font-weight: 600;
+  padding: 0 0.5rem;
+}
+
+.month-nav .nav-link {
+  color: var(--accent);
+  text-decoration: none;
+  padding: 0.25rem 0.5rem;
+  border: 1px solid var(--rule);
+  border-radius: 3px;
+  background: #fff;
+  font-size: 0.9rem;
+}
+
+.month-nav .nav-link:hover {
+  background: #f0ece0;
+}
+
+.month-nav .spacer {
+  flex: 1;
+}
+
+.month-nav .month-picker {
+  padding: 0.3rem 0.5rem;
+  font-size: 0.9rem;
+}
+
+.total .divider {
+  color: var(--muted);
+  margin: 0 0.25rem;
+  font-weight: 400;
+}
+
+.total .planned {
+  color: var(--muted);
+}
+
+table.month-entries thead th {
+  text-align: left;
+  padding: 0.25rem 0.5rem;
+  font-size: 0.8rem;
+  color: var(--muted);
+  font-weight: 500;
+  border-bottom: 1px solid var(--rule);
+}
+
+table.month-entries th.col-planned,
+table.month-entries th.col-applied {
+  text-align: right;
+  width: 8rem;
+}
+
+table.month-entries th.col-actions {
+  width: 2.25rem;
+}
+
+table.month-entries td.entry-name input[name="name"] {
+  width: 100%;
+  min-width: 8rem;
+}
+
+table.month-entries td.entry-amount input {
+  width: 100%;
+  text-align: right;
+}
+
+tr.entry.state-edited {
+  background: #fff7e0;
+}
+
+tr.entry.state-new_in_month {
+  background: #e8f2ff;
+}
+
+.tag {
+  display: inline-block;
+  margin-left: 0.4rem;
+  padding: 0.05rem 0.4rem;
+  border-radius: 3px;
+  font-size: 0.72rem;
+  font-weight: 600;
+  text-transform: lowercase;
+  letter-spacing: 0.02em;
+}
+
+.tag-edited {
+  background: #f4d07a;
+  color: #5a3f00;
+}
+
+.tag-new {
+  background: #9cc5ef;
+  color: #123a66;
+}
+
+.month-add-form {
+  grid-template-columns: 1fr 9rem auto;
+}
+
+.month-missing-body {
+  padding: 0.75rem 0.5rem;
+  color: var(--muted);
+}
+
+.month-create-form {
+  padding: 0.25rem 0.5rem 0.75rem;
+}
diff --git a/src/quartermaster/templates/index.html b/src/quartermaster/templates/index.html
index ec46dc5..4d48a18 100644
--- a/src/quartermaster/templates/index.html
+++ b/src/quartermaster/templates/index.html
@@ -1,6 +1,23 @@
 {% extends "base.html" %}
 {% block content %}
 <div class="budget">
+  <nav class="month-nav budget-nav">
+    <span class="month-label">Budget configuration</span>
+    <span class="spacer"></span>
+    <a class="nav-link" href="/month/{{ current_year_month }}">This month ({{ current_year_month }})</a>
+    {% if all_months %}
+      <select
+        class="month-picker"
+        onchange="if(this.value){window.location=this.value}"
+        aria-label="Jump to month"
+      >
+        <option value="">Jump to...</option>
+        {% for m in all_months %}
+          <option value="/month/{{ m.year_month }}">{{ m.year_month }}</option>
+        {% endfor %}
+      </select>
+    {% endif %}
+  </nav>
   {% for section in sections %}
     {% if section.section.value == 'debt_minimum' %}
       {% include "partials/section.html" %}
diff --git a/src/quartermaster/templates/month.html b/src/quartermaster/templates/month.html
new file mode 100644
index 0000000..91d2475
--- /dev/null
+++ b/src/quartermaster/templates/month.html
@@ -0,0 +1,14 @@
+{% extends "base.html" %}
+{% block content %}
+<div class="budget">
+  {% include "partials/month_nav.html" %}
+  {% for section in sections %}
+    {% if section.section.value == 'debt_minimum' %}
+      {% include "partials/month_section.html" %}
+      {% include "partials/month_target.html" %}
+    {% else %}
+      {% include "partials/month_section.html" %}
+    {% endif %}
+  {% endfor %}
+</div>
+{% endblock %}
diff --git a/src/quartermaster/templates/month_create.html b/src/quartermaster/templates/month_create.html
new file mode 100644
index 0000000..5262a8a
--- /dev/null
+++ b/src/quartermaster/templates/month_create.html
@@ -0,0 +1,23 @@
+{% extends "base.html" %}
+{% block content %}
+<div class="budget">
+  {% include "partials/month_nav.html" %}
+  <section class="section month-missing">
+    <div class="section-header">
+      <h2>No snapshot yet</h2>
+    </div>
+    <p class="month-missing-body">
+      This month has not been created. Creating it will snapshot the current
+      budget: every entry, its planned amount, and the current Primary Debt
+      Target. Applied amounts start at $0.00.
+    </p>
+    <form
+      hx-post="/month/{{ year_month }}/create"
+      hx-swap="none"
+      class="month-create-form"
+    >
+      <button type="submit">Create {{ year_month }}</button>
+    </form>
+  </section>
+</div>
+{% endblock %}
diff --git a/src/quartermaster/templates/partials/month_nav.html b/src/quartermaster/templates/partials/month_nav.html
new file mode 100644
index 0000000..3d3c636
--- /dev/null
+++ b/src/quartermaster/templates/partials/month_nav.html
@@ -0,0 +1,22 @@
+<nav class="month-nav">
+  <a class="nav-link" href="/month/{{ prev_year_month }}" aria-label="Previous month">&larr; {{ prev_year_month }}</a>
+  <span class="month-label">{{ year_month }}</span>
+  <a class="nav-link" href="/month/{{ next_year_month }}" aria-label="Next month">{{ next_year_month }} &rarr;</a>
+  <span class="spacer"></span>
+  {% if all_months %}
+    <select
+      class="month-picker"
+      onchange="if(this.value){window.location=this.value}"
+      aria-label="Jump to month"
+    >
+      <option value="">Jump to...</option>
+      {% for m in all_months %}
+        <option
+          value="/month/{{ m.year_month }}"
+          {% if m.year_month == year_month %}selected{% endif %}
+        >{{ m.year_month }}</option>
+      {% endfor %}
+    </select>
+  {% endif %}
+  <a class="nav-link" href="/">Budget config</a>
+</nav>
diff --git a/src/quartermaster/templates/partials/month_section.html b/src/quartermaster/templates/partials/month_section.html
new file mode 100644
index 0000000..922316d
--- /dev/null
+++ b/src/quartermaster/templates/partials/month_section.html
@@ -0,0 +1,91 @@
+<section class="section" id="section-{{ section.section.value }}">
+  <div class="section-header">
+    <h2>{{ section.label }}</h2>
+    <span class="total" data-testid="total-{{ section.section.value }}">
+      <span class="applied">${{ '%.2f' | format(section.total_applied) }}</span>
+      <span class="divider">/</span>
+      <span class="planned">${{ '%.2f' | format(section.total_planned) }}</span>
+    </span>
+  </div>
+  <table class="entries month-entries">
+    <thead>
+      <tr>
+        <th class="col-name">Name</th>
+        <th class="col-planned">Planned</th>
+        <th class="col-applied">Applied</th>
+        <th class="col-actions"></th>
+      </tr>
+    </thead>
+    <tbody>
+      {% for row in section.rows %}
+        <tr class="entry state-{{ row.state.value }}" data-entry-id="{{ row.entry.id }}">
+          <td class="entry-name">
+            <input
+              type="text"
+              name="name"
+              value="{{ row.entry.name }}"
+              hx-post="/month/{{ month.year_month }}/entries/{{ row.entry.id }}"
+              hx-trigger="change"
+              hx-target="#section-{{ section.section.value }}"
+              hx-swap="outerHTML"
+            >
+            {% if row.state.value == 'edited' %}
+              <span class="tag tag-edited">modified</span>
+            {% elif row.state.value == 'new_in_month' %}
+              <span class="tag tag-new">new this month</span>
+            {% endif %}
+          </td>
+          <td class="entry-amount">
+            <input
+              type="number" step="0.01" min="0"
+              name="planned"
+              value="{{ '%.2f' | format(row.entry.planned) }}"
+              hx-post="/month/{{ month.year_month }}/entries/{{ row.entry.id }}"
+              hx-trigger="change"
+              hx-target="#section-{{ section.section.value }}"
+              hx-swap="outerHTML"
+            >
+          </td>
+          <td class="entry-amount">
+            <input
+              type="number" step="0.01" min="0"
+              name="applied"
+              value="{{ '%.2f' | format(row.entry.applied) }}"
+              hx-post="/month/{{ month.year_month }}/entries/{{ row.entry.id }}"
+              hx-trigger="change"
+              hx-target="#section-{{ section.section.value }}"
+              hx-swap="outerHTML"
+            >
+          </td>
+          <td class="entry-actions">
+            <button
+              class="delete"
+              type="button"
+              hx-delete="/month/{{ month.year_month }}/entries/{{ row.entry.id }}"
+              hx-target="#section-{{ section.section.value }}"
+              hx-swap="outerHTML"
+              aria-label="Delete {{ row.entry.name }}"
+            >&times;</button>
+          </td>
+        </tr>
+      {% else %}
+        <tr class="empty"><td colspan="4">No entries.</td></tr>
+      {% endfor %}
+      <tr class="add-row">
+        <td colspan="4">
+          <form
+            class="add-form month-add-form"
+            hx-post="/month/{{ month.year_month }}/sections/{{ section.section.value }}/entries"
+            hx-target="#section-{{ section.section.value }}"
+            hx-swap="outerHTML"
+            hx-on::after-request="if(event.detail.successful) this.reset()"
+          >
+            <input type="text" name="name" placeholder="Name" required>
+            <input type="number" name="planned" step="0.01" min="0" placeholder="Planned" required>
+            <button type="submit">Add</button>
+          </form>
+        </td>
+      </tr>
+    </tbody>
+  </table>
+</section>
diff --git a/src/quartermaster/templates/partials/month_target.html b/src/quartermaster/templates/partials/month_target.html
new file mode 100644
index 0000000..5f0b73a
--- /dev/null
+++ b/src/quartermaster/templates/partials/month_target.html
@@ -0,0 +1,46 @@
+<section class="section target-section" id="section-debt_target" hx-swap-oob="outerHTML">
+  <div class="section-header">
+    <h2>Primary Debt Target</h2>
+    {% if target.entry %}
+      <span class="total">
+        <span class="applied">${{ '%.2f' | format(target.entry.applied) }}</span>
+        <span class="divider">/</span>
+        <span class="planned">${{ '%.2f' | format(target.entry.planned) }}</span>
+      </span>
+    {% else %}
+      <span class="total empty">$0.00</span>
+    {% endif %}
+  </div>
+  <table class="entries">
+    <tbody>
+      <tr class="entry">
+        <td class="entry-name">
+          {% if target.entry %}{{ target.entry.name }}{% else %}<span class="muted">No target selected.</span>{% endif %}
+        </td>
+        <td class="entry-amount"></td>
+        <td class="entry-actions"></td>
+      </tr>
+      <tr class="add-row">
+        <td colspan="3">
+          <form
+            class="target-form"
+            hx-post="/month/{{ month.year_month }}/target"
+            hx-target="#section-debt_target"
+            hx-swap="outerHTML"
+          >
+            <select name="month_entry_id">
+              <option value="">(none)</option>
+              {% for dm in debt_minimums %}
+                <option
+                  value="{{ dm.id }}"
+                  {% if target.month_entry_id == dm.id %}selected{% endif %}
+                >{{ dm.name }}: ${{ '%.2f' | format(dm.planned) }}</option>
+              {% endfor %}
+            </select>
+            <button type="submit">Set</button>
+          </form>
+        </td>
+      </tr>
+    </tbody>
+  </table>
+</section>