diff --git a/src/quartermaster/month_service.py b/src/quartermaster/month_service.py
new file mode 100644
index 0000000..7c96387
--- /dev/null
+++ b/src/quartermaster/month_service.py
@@ -0,0 +1,228 @@
+from __future__ import annotations
+
+import re
+from dataclasses import dataclass
+from datetime import date
+from decimal import Decimal
+from enum import Enum
+
+from sqlalchemy import select
+from sqlalchemy.orm import Session
+
+from quartermaster.models import (
+    DebtTarget,
+    Entry,
+    Month,
+    MonthDebtTarget,
+    MonthEntry,
+    Section,
+)
+
+YEAR_MONTH_RE = re.compile(r"^\d{4}-(0[1-9]|1[0-2])$")
+
+
+class DeviationState(str, Enum):
+    unchanged = "unchanged"
+    edited = "edited"
+    new_in_month = "new_in_month"
+
+
+@dataclass(frozen=True)
+class MonthRow:
+    entry: MonthEntry
+    state: DeviationState
+
+
+@dataclass(frozen=True)
+class MonthSectionView:
+    section: Section
+    label: str
+    rows: list[MonthRow]
+    total_planned: Decimal
+    total_applied: Decimal
+
+
+def valid_year_month(year_month: str) -> bool:
+    return bool(YEAR_MONTH_RE.match(year_month))
+
+
+def current_year_month() -> str:
+    today = date.today()
+    return f"{today.year:04d}-{today.month:02d}"
+
+
+def shift_year_month(year_month: str, delta: int) -> str:
+    year, month = (int(part) for part in year_month.split("-"))
+    index = (year * 12 + (month - 1)) + delta
+    new_year, new_month0 = divmod(index, 12)
+    return f"{new_year:04d}-{new_month0 + 1:02d}"
+
+
+def get_month(db: Session, year_month: str) -> Month | None:
+    stmt = select(Month).where(Month.year_month == year_month)
+    return db.scalar(stmt)
+
+
+def list_months(db: Session) -> list[Month]:
+    stmt = select(Month).order_by(Month.year_month)
+    return list(db.scalars(stmt))
+
+
+def create_month(db: Session, year_month: str) -> Month:
+    if not valid_year_month(year_month):
+        raise ValueError("year_month must be formatted as YYYY-MM")
+    existing = get_month(db, year_month)
+    if existing is not None:
+        return existing
+
+    month = Month(year_month=year_month)
+    db.add(month)
+    db.flush()
+
+    budget_entries = list(db.scalars(select(Entry).order_by(Entry.id)))
+    source_to_month_entry: dict[int, MonthEntry] = {}
+    for e in budget_entries:
+        month_entry = MonthEntry(
+            month_id=month.id,
+            section=e.section,
+            name=e.name,
+            planned=e.amount,
+            applied=Decimal("0.00"),
+            origin_name=e.name,
+            origin_planned=e.amount,
+            source_entry_id=e.id,
+        )
+        db.add(month_entry)
+        source_to_month_entry[e.id] = month_entry
+
+    db.flush()
+
+    budget_target = db.get(DebtTarget, 1)
+    target_entry_id: int | None = None
+    if budget_target is not None and budget_target.debt_minimum_id is not None:
+        mapped = source_to_month_entry.get(budget_target.debt_minimum_id)
+        if mapped is not None:
+            target_entry_id = mapped.id
+
+    db.add(
+        MonthDebtTarget(month_id=month.id, month_entry_id=target_entry_id)
+    )
+    db.commit()
+    db.refresh(month)
+    return month
+
+
+def deviation_state(entry: MonthEntry) -> DeviationState:
+    if entry.origin_name is None or entry.origin_planned is None:
+        return DeviationState.new_in_month
+    if entry.name != entry.origin_name or entry.planned != entry.origin_planned:
+        return DeviationState.edited
+    return DeviationState.unchanged
+
+
+def _rows(entries: list[MonthEntry]) -> list[MonthRow]:
+    return [MonthRow(entry=e, state=deviation_state(e)) for e in entries]
+
+
+def section_view(month: Month, section: Section, label: str) -> MonthSectionView:
+    entries = [e for e in month.entries if e.section == section]
+    entries.sort(key=lambda e: e.id)
+    rows = _rows(entries)
+    total_planned = sum((e.planned for e in entries), Decimal("0"))
+    total_applied = sum((e.applied for e in entries), Decimal("0"))
+    return MonthSectionView(
+        section=section,
+        label=label,
+        rows=rows,
+        total_planned=total_planned,
+        total_applied=total_applied,
+    )
+
+
+def add_month_entry(
+    db: Session,
+    month: Month,
+    section: Section,
+    name: str,
+    planned: Decimal,
+) -> MonthEntry:
+    entry = MonthEntry(
+        month_id=month.id,
+        section=section,
+        name=name.strip(),
+        planned=planned,
+        applied=Decimal("0.00"),
+        origin_name=None,
+        origin_planned=None,
+        source_entry_id=None,
+    )
+    db.add(entry)
+    db.commit()
+    db.refresh(entry)
+    return entry
+
+
+def get_month_entry(db: Session, month: Month, entry_id: int) -> MonthEntry | None:
+    entry = db.get(MonthEntry, entry_id)
+    if entry is None or entry.month_id != month.id:
+        return None
+    return entry
+
+
+def delete_month_entry(db: Session, month: Month, entry_id: int) -> Section | None:
+    entry = get_month_entry(db, month, entry_id)
+    if entry is None:
+        return None
+    section = entry.section
+    db.delete(entry)
+    db.commit()
+    return section
+
+
+def update_month_entry(
+    db: Session,
+    month: Month,
+    entry_id: int,
+    *,
+    name: str | None = None,
+    planned: Decimal | None = None,
+    applied: Decimal | None = None,
+) -> MonthEntry | None:
+    entry = get_month_entry(db, month, entry_id)
+    if entry is None:
+        return None
+    if name is not None:
+        entry.name = name.strip()
+    if planned is not None:
+        entry.planned = planned
+    if applied is not None:
+        entry.applied = applied
+    db.commit()
+    db.refresh(entry)
+    return entry
+
+
+def get_month_target(db: Session, month: Month) -> MonthDebtTarget:
+    if month.target is not None:
+        return month.target
+    target = MonthDebtTarget(month_id=month.id, month_entry_id=None)
+    db.add(target)
+    db.commit()
+    db.refresh(target)
+    return target
+
+
+def set_month_target(
+    db: Session, month: Month, month_entry_id: int | None
+) -> MonthDebtTarget:
+    target = get_month_target(db, month)
+    if month_entry_id is not None:
+        candidate = get_month_entry(db, month, month_entry_id)
+        if candidate is None or candidate.section != Section.debt_minimum:
+            raise ValueError(
+                "month_entry_id must reference a debt minimum entry in this month"
+            )
+    target.month_entry_id = month_entry_id
+    db.commit()
+    db.refresh(target)
+    return target