diff --git a/crates/ledger-core/src/payroll.rs b/crates/ledger-core/src/payroll.rs
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+++ b/crates/ledger-core/src/payroll.rs
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+//! Payroll: accounting-only payroll runs (no tax tables).
+//!
+//! A payroll run posts one balanced entry: gross wages hit Salary & Wage
+//! Expense, the employer share of FICA hits Payroll Tax Expense, employee
+//! withholdings and deductions accumulate in the 2110–2150 payable accounts,
+//! and net pay leaves cash. A later [`crate::Ledger::remit_payroll_taxes`]
+//! clears the payables when the tax payments go out.
+//!
+//! There is no tax law here: every withholding number is supplied by the
+//! caller. All amounts are exact [`rust_decimal::Decimal`], never floats.
+
+use crate::{AccountId, Currency, EntryId, LedgerError, Money, Side};
+use chrono::NaiveDate;
+use rust_decimal::Decimal;
+use serde::{Deserialize, Serialize};
+
+/// Opaque payroll run handle. Created by [`crate::Ledger`]; never forged.
+pub type PayrollRunId = u64;
+
+/// One employee's payroll line: gross pay and every withholding and
+/// deduction. All amounts must be non-negative; the engine rejects
+/// negatives, and rejects the line when withholdings exceed gross.
+#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
+pub struct PayrollLine {
+    /// Employee name, for the run record.
+    pub employee_name: String,
+    /// Gross pay for the period.
+    pub gross_pay: Decimal,
+    /// Federal income tax withheld.
+    pub federal_withholding: Decimal,
+    /// State income tax withheld.
+    pub state_withholding: Decimal,
+    /// Employee share of Social Security tax.
+    pub ss_employee: Decimal,
+    /// Employee share of Medicare tax.
+    pub medicare_employee: Decimal,
+    /// Employer share of Social Security tax.
+    pub ss_employer: Decimal,
+    /// Employer share of Medicare tax.
+    pub medicare_employer: Decimal,
+    /// Other deductions (retirement, insurance, garnishments, ...).
+    pub other_deductions: Decimal,
+}
+
+impl PayrollLine {
+    /// Gross minus all employee withholdings and deductions. Must be >= 0:
+    /// you cannot withhold more than the employee earned.
+    pub fn net_pay(&self) -> Decimal {
+        self.gross_pay
+            - self.federal_withholding
+            - self.state_withholding
+            - self.ss_employee
+            - self.medicare_employee
+            - self.other_deductions
+    }
+}
+
+/// A posted payroll run: the inputs plus the journal entry it posted.
+#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
+pub struct PayrollRun {
+    /// Run id (the allocator's).
+    pub id: PayrollRunId,
+    /// Pay date: the date of the posted entry.
+    pub date: NaiveDate,
+    /// One line per employee.
+    pub lines: Vec<PayrollLine>,
+    /// The posted journal entry.
+    pub entry_id: EntryId,
+}
+
+/// The chart accounts a payroll run posts to, resolved by code.
+#[derive(Clone, Copy, Debug)]
+pub struct PayrollAccounts {
+    /// `6100` Salary & Wage Expense.
+    pub salary_expense: AccountId,
+    /// `6110` Payroll Tax Expense.
+    pub payroll_tax_expense: AccountId,
+    /// `2110` Federal Withholding Payable.
+    pub federal_payable: AccountId,
+    /// `2120` State Withholding Payable.
+    pub state_payable: AccountId,
+    /// `2130` SS Tax Payable.
+    pub ss_payable: AccountId,
+    /// `2140` Medicare Tax Payable.
+    pub medicare_payable: AccountId,
+    /// `2150` Other Deductions Payable.
+    pub other_payable: AccountId,
+}
+
+/// Validate a payroll run's lines without mutating: non-empty, no negative
+/// amounts, no negative net pay. Journaling stores run this before writing
+/// anything, so a doomed run never leaves journal lines behind.
+pub fn validate_payroll_lines(lines: &[PayrollLine]) -> Result<(), LedgerError> {
+    if lines.is_empty() {
+        return Err(LedgerError::EmptyPayroll);
+    }
+    for line in lines {
+        for amount in [
+            line.gross_pay,
+            line.federal_withholding,
+            line.state_withholding,
+            line.ss_employee,
+            line.medicare_employee,
+            line.ss_employer,
+            line.medicare_employer,
+            line.other_deductions,
+        ] {
+            if amount < Decimal::ZERO {
+                return Err(LedgerError::NegativePayrollAmount(amount));
+            }
+        }
+        if line.net_pay() < Decimal::ZERO {
+            return Err(LedgerError::NegativeNetPay(line.employee_name.clone()));
+        }
+    }
+    Ok(())
+}
+
+/// The auto-posted payroll entry's lines, in posting order: Dr 6100 gross,
+/// Dr 6110 the employer FICA share, Cr 2110–2150 the payables, Cr cash net
+/// pay. Zero totals are skipped — the engine rejects zero-amount lines.
+///
+/// Shared by [`crate::Ledger::run_payroll`] and the journaling store, so the
+/// journaled entry and the posted entry are built from the same math. The
+/// plan always balances: debits are gross + employer FICA, credits are the
+/// payables + net, and net is defined as gross minus employee withholdings.
+pub fn payroll_entry_plan(
+    lines: &[PayrollLine],
+    accounts: PayrollAccounts,
+    cash_account: AccountId,
+    base: Currency,
+) -> Vec<(AccountId, Side, Money)> {
+    let sum = |f: fn(&PayrollLine) -> Decimal| lines.iter().map(f).sum::<Decimal>();
+    let gross = sum(|l| l.gross_pay);
+    let employer = sum(|l| l.ss_employer + l.medicare_employer);
+    let federal = sum(|l| l.federal_withholding);
+    let state = sum(|l| l.state_withholding);
+    let ss = sum(|l| l.ss_employee + l.ss_employer);
+    let medicare = sum(|l| l.medicare_employee + l.medicare_employer);
+    let other = sum(|l| l.other_deductions);
+    let net = sum(|l| l.net_pay());
+    let mut plan = Vec::new();
+    let mut push = |account: AccountId, side: Side, amount: Decimal| {
+        if !amount.is_zero() {
+            plan.push((account, side, Money::new(amount, base)));
+        }
+    };
+    push(accounts.salary_expense, Side::Debit, gross);
+    push(accounts.payroll_tax_expense, Side::Debit, employer);
+    push(accounts.federal_payable, Side::Credit, federal);
+    push(accounts.state_payable, Side::Credit, state);
+    push(accounts.ss_payable, Side::Credit, ss);
+    push(accounts.medicare_payable, Side::Credit, medicare);
+    push(accounts.other_payable, Side::Credit, other);
+    push(cash_account, Side::Credit, net);
+    plan
+}
+
+/// The remittance entry's lines: Dr each payable with a non-zero balance,
+/// Cr cash for the total. Balances are normal-side (positive = owed), so
+/// the plan balances by construction.
+pub fn payroll_remit_plan(
+    payables: &[(AccountId, Decimal)],
+    cash_account: AccountId,
+    base: Currency,
+) -> Vec<(AccountId, Side, Money)> {
+    let mut plan = Vec::new();
+    let mut total = Decimal::ZERO;
+    for (account, balance) in payables {
+        if balance.is_zero() {
+            continue;
+        }
+        plan.push((*account, Side::Debit, Money::new(*balance, base)));
+        total += *balance;
+    }
+    if !total.is_zero() {
+        plan.push((cash_account, Side::Credit, Money::new(total, base)));
+    }
+    plan
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::{AccountType, Ledger};
+
+    fn d(cents: i64) -> Decimal {
+        Decimal::new(cents, 2)
+    }
+
+    fn line(name: &str) -> PayrollLine {
+        PayrollLine {
+            employee_name: name.to_string(),
+            gross_pay: d(500_000),          // 5000.00
+            federal_withholding: d(75_000), // 750.00
+            state_withholding: d(25_000),   // 250.00
+            ss_employee: d(31_000),         // 310.00
+            medicare_employee: d(7_250),    // 72.50
+            ss_employer: d(31_000),
+            medicare_employer: d(7_250),
+            other_deductions: d(10_000), // 100.00
+        }
+    }
+
+    fn ledger_with_cash() -> (Ledger, AccountId) {
+        let mut ledger = Ledger::new(Currency::USD);
+        let cash = ledger
+            .add_account("1000", "Cash", AccountType::Asset, None, false)
+            .unwrap();
+        (ledger, cash)
+    }
+
+    #[test]
+    fn net_pay_math() {
+        let l = line("Ada");
+        // 5000 - 750 - 250 - 310 - 72.50 - 100 = 3517.50
+        assert_eq!(l.net_pay(), d(351_750));
+    }
+
+    #[test]
+    fn validation_rejects_empty_negative_and_overwithheld() {
+        assert_eq!(validate_payroll_lines(&[]), Err(LedgerError::EmptyPayroll));
+        let mut bad = line("Bob");
+        bad.ss_employee = d(-1);
+        assert_eq!(
+            validate_payroll_lines(std::slice::from_ref(&bad)),
+            Err(LedgerError::NegativePayrollAmount(d(-1)))
+        );
+        let mut over = line("Cat");
+        over.other_deductions = d(99_999_999);
+        assert!(over.net_pay() < Decimal::ZERO);
+        assert_eq!(
+            validate_payroll_lines(std::slice::from_ref(&over)),
+            Err(LedgerError::NegativeNetPay("Cat".to_string()))
+        );
+        assert!(validate_payroll_lines(&[line("Ada")]).is_ok());
+    }
+
+    #[test]
+    fn entry_plan_balances_and_skips_zeros() {
+        let cash = ledger_with_cash().1;
+        let accounts = PayrollAccounts {
+            salary_expense: 101,
+            payroll_tax_expense: 102,
+            federal_payable: 103,
+            state_payable: 104,
+            ss_payable: 105,
+            medicare_payable: 106,
+            other_payable: 107,
+        };
+        // No state withholding and no other deductions: those credits vanish.
+        let mut l = line("Ada");
+        l.state_withholding = Decimal::ZERO;
+        l.other_deductions = Decimal::ZERO;
+        let plan = payroll_entry_plan(&[l], accounts, cash, Currency::USD);
+        assert_eq!(plan.len(), 6); // gross, employer tax, fed, ss, medicare, cash
+        let mut debits = Decimal::ZERO;
+        let mut credits = Decimal::ZERO;
+        for (_, side, amount) in &plan {
+            match side {
+                Side::Debit => debits += amount.amount,
+                Side::Credit => credits += amount.amount,
+            }
+        }
+        assert_eq!(debits, credits);
+        assert!(debits > Decimal::ZERO);
+        for (_, _, amount) in &plan {
+            assert!(!amount.amount.is_zero());
+        }
+    }
+
+    #[test]
+    fn run_payroll_posts_balanced_entry_and_records_run() {
+        let (mut ledger, cash) = ledger_with_cash();
+        let date = NaiveDate::from_ymd_opt(2026, 10, 9).unwrap();
+        let id = ledger
+            .run_payroll(date, vec![line("Ada"), line("Bob")], cash)
+            .unwrap();
+        assert_eq!(id, 1);
+        assert_eq!(ledger.next_payroll_run_id(), 2);
+        let run = ledger.get_payroll_run(id).unwrap();
+        assert_eq!(run.date, date);
+        assert_eq!(run.lines.len(), 2);
+        let entry = ledger.get_entry(run.entry_id).unwrap();
+        assert_eq!(entry.status(), crate::EntryStatus::Posted);
+        // Debits equal credits, exactly.
+        let mut debits = Decimal::ZERO;
+        let mut credits = Decimal::ZERO;
+        for l in entry.lines() {
+            match l.side {
+                Side::Debit => debits += l.amount.amount,
+                Side::Credit => credits += l.amount.amount,
+            }
+        }
+        assert_eq!(debits, credits);
+        // Gross x2 = 10000 hits Salary & Wage Expense (debit-normal).
+        let salary = ledger.chart().find_by_code("6100").unwrap().id;
+        assert_eq!(ledger.account_balance(salary).unwrap(), d(1_000_000));
+        // Net x2 = 7035 leaves cash (debit-normal): -7035.
+        assert_eq!(ledger.account_balance(cash).unwrap(), -d(703_500));
+        // The seven payroll accounts were auto-created.
+        for code in ["6100", "6110", "2110", "2120", "2130", "2140", "2150"] {
+            assert!(
+                ledger.chart().find_by_code(code).is_some(),
+                "missing {code}"
+            );
+        }
+        // Runs list in id order.
+        let id2 = ledger.run_payroll(date, vec![line("Cat")], cash).unwrap();
+        let ids: Vec<_> = ledger.list_payroll_runs().iter().map(|r| r.id).collect();
+        assert_eq!(ids, vec![id, id2]);
+    }
+
+    #[test]
+    fn run_payroll_rejects_closed_period() {
+        let (mut ledger, cash) = ledger_with_cash();
+        let date = NaiveDate::from_ymd_opt(2026, 9, 30).unwrap();
+        let pid = ledger
+            .create_period("Sep", NaiveDate::from_ymd_opt(2026, 9, 1).unwrap(), date)
+            .unwrap();
+        ledger.close_period(pid).unwrap();
+        let err = ledger.run_payroll(date, vec![line("Ada")], cash);
+        assert_eq!(err, Err(LedgerError::ClosedPeriod(pid, date.to_string())));
+    }
+
+    #[test]
+    fn run_payroll_rejects_bad_inputs() {
+        let (mut ledger, cash) = ledger_with_cash();
+        let date = NaiveDate::from_ymd_opt(2026, 10, 9).unwrap();
+        assert_eq!(
+            ledger.run_payroll(date, vec![], cash),
+            Err(LedgerError::EmptyPayroll)
+        );
+        let mut bad = line("Ada");
+        bad.gross_pay = d(-100);
+        assert_eq!(
+            ledger.run_payroll(date, vec![bad], cash),
+            Err(LedgerError::NegativePayrollAmount(d(-100)))
+        );
+        assert_eq!(
+            ledger.run_payroll(date, vec![line("Ada")], 999),
+            Err(LedgerError::UnknownAccount(999))
+        );
+    }
+
+    #[test]
+    fn remit_clears_payables() {
+        let (mut ledger, cash) = ledger_with_cash();
+        let date = NaiveDate::from_ymd_opt(2026, 10, 9).unwrap();
+        ledger.run_payroll(date, vec![line("Ada")], cash).unwrap();
+        // Withholdings owed: 750 + 250 + (310+310) + (72.50+72.50) + 100 = 1865.
+        let fed = ledger.chart().find_by_code("2110").unwrap().id;
+        assert_eq!(ledger.account_balance(fed).unwrap(), d(75_000));
+        // Nothing owed before any payroll: fresh ledger remits nothing.
+        let (mut fresh, fresh_cash) = ledger_with_cash();
+        assert_eq!(
+            fresh.remit_payroll_taxes(date, fresh_cash),
+            Err(LedgerError::NothingToRemit)
+        );
+        // Remit: payables go to zero, cash drops by the total.
+        let cash_before = ledger.account_balance(cash).unwrap();
+        let entry = ledger.remit_payroll_taxes(date, cash).unwrap();
+        let posted = ledger.get_entry(entry).unwrap();
+        assert_eq!(posted.status(), crate::EntryStatus::Posted);
+        for code in ["2110", "2120", "2130", "2140", "2150"] {
+            let id = ledger.chart().find_by_code(code).unwrap().id;
+            assert_eq!(
+                ledger.account_balance(id).unwrap(),
+                Decimal::ZERO,
+                "payable {code} not cleared"
+            );
+        }
+        assert_eq!(
+            ledger.account_balance(cash).unwrap(),
+            cash_before - d(186_500)
+        );
+        // Second remit finds nothing left.
+        assert_eq!(
+            ledger.remit_payroll_taxes(date, cash),
+            Err(LedgerError::NothingToRemit)
+        );
+    }
+
+    #[test]
+    fn remit_rejects_closed_period() {
+        let (mut ledger, cash) = ledger_with_cash();
+        let date = NaiveDate::from_ymd_opt(2026, 9, 30).unwrap();
+        ledger
+            .run_payroll(
+                NaiveDate::from_ymd_opt(2026, 9, 15).unwrap(),
+                vec![line("Ada")],
+                cash,
+            )
+            .unwrap();
+        let pid = ledger
+            .create_period("Sep", NaiveDate::from_ymd_opt(2026, 9, 1).unwrap(), date)
+            .unwrap();
+        ledger.close_period(pid).unwrap();
+        assert_eq!(
+            ledger.remit_payroll_taxes(date, cash),
+            Err(LedgerError::ClosedPeriod(pid, date.to_string()))
+        );
+    }
+
+    #[test]
+    fn replay_payroll_run_checks_id() {
+        let (mut ledger, _cash) = ledger_with_cash();
+        let date = NaiveDate::from_ymd_opt(2026, 10, 9).unwrap();
+        let entry = ledger.create_entry(date, "x");
+        let err = ledger.replay_payroll_run(7, date, vec![line("Ada")], entry);
+        assert_eq!(err, Err(LedgerError::ReplayIdMismatch(7, 1)));
+        // The allocator did not move on failure.
+        assert_eq!(ledger.next_payroll_run_id(), 1);
+        ledger
+            .replay_payroll_run(1, date, vec![line("Ada")], entry)
+            .unwrap();
+        assert_eq!(ledger.next_payroll_run_id(), 2);
+        assert!(ledger.get_payroll_run(1).is_some());
+        assert!(ledger.get_payroll_run(2).is_none());
+    }
+}