diff --git a/client/src/pages/MonthlySummary.jsx b/client/src/pages/MonthlySummary.jsx
index 264bf7c..5d16779 100644
--- a/client/src/pages/MonthlySummary.jsx
+++ b/client/src/pages/MonthlySummary.jsx
@@ -1,5 +1,274 @@
-function MonthlySummary() {
-  return <div><h1>Monthly Summary</h1><p>Placeholder — coming soon.</p></div>;
+import { useState, useEffect } from 'react';
+
+const MONTH_NAMES = [
+  'January', 'February', 'March', 'April', 'May', 'June',
+  'July', 'August', 'September', 'October', 'November', 'December',
+];
+
+function formatCurrency(value) {
+  const num = parseFloat(value) || 0;
+  return '$' + num.toLocaleString('en-US', { minimumFractionDigits: 2, maximumFractionDigits: 2 });
 }
 
+function StatCard({ label, value, valueColor }) {
+  return (
+    <div style={styles.card}>
+      <div style={styles.cardLabel}>{label}</div>
+      <div style={{ ...styles.cardValue, color: valueColor || '#222' }}>{value}</div>
+    </div>
+  );
+}
+
+function MonthlySummary() {
+  const now = new Date();
+  const [year, setYear] = useState(now.getFullYear());
+  const [month, setMonth] = useState(now.getMonth() + 1);
+  const [data, setData] = useState(null);
+  const [loading, setLoading] = useState(false);
+  const [error, setError] = useState(null);
+
+  useEffect(() => {
+    loadSummary(year, month);
+  }, [year, month]);
+
+  async function loadSummary(y, m) {
+    setLoading(true);
+    setError(null);
+    try {
+      const res = await fetch(`/api/summary/monthly?year=${y}&month=${m}`);
+      if (!res.ok) throw new Error(`Server error: ${res.status}`);
+      const json = await res.json();
+      setData(json);
+    } catch (err) {
+      setError(err.message);
+    } finally {
+      setLoading(false);
+    }
+  }
+
+  function prevMonth() {
+    if (month === 1) {
+      setYear(y => y - 1);
+      setMonth(12);
+    } else {
+      setMonth(m => m - 1);
+    }
+  }
+
+  function nextMonth() {
+    if (month === 12) {
+      setYear(y => y + 1);
+      setMonth(1);
+    } else {
+      setMonth(m => m + 1);
+    }
+  }
+
+  const surplusColor = data && data.summary.surplus_deficit >= 0 ? '#2a7a2a' : '#c0392b';
+
+  return (
+    <div style={styles.container}>
+      <div style={styles.monthNav}>
+        <button style={styles.navButton} onClick={prevMonth}>&larr;</button>
+        <span style={styles.monthLabel}>{MONTH_NAMES[month - 1]} {year}</span>
+        <button style={styles.navButton} onClick={nextMonth}>&rarr;</button>
+      </div>
+
+      {error && (
+        <div style={styles.errorBanner}>Error: {error}</div>
+      )}
+
+      {loading && (
+        <div style={styles.loadingMsg}>Loading...</div>
+      )}
+
+      {!loading && data && (
+        <>
+          <div style={styles.cardRow}>
+            <StatCard
+              label="Total Income (net)"
+              value={formatCurrency(data.income.net)}
+            />
+            <StatCard
+              label="Total Bills Planned"
+              value={formatCurrency(data.bills.planned)}
+            />
+            <StatCard
+              label="Total Variable Spending"
+              value={formatCurrency(data.actuals.total)}
+            />
+            <StatCard
+              label="Total One-time"
+              value={formatCurrency(data.one_time_expenses.total)}
+            />
+          </div>
+
+          <div style={styles.cardRow}>
+            <StatCard
+              label="Total Spending"
+              value={formatCurrency(data.summary.total_spending)}
+            />
+            <StatCard
+              label="Surplus / Deficit"
+              value={formatCurrency(data.summary.surplus_deficit)}
+              valueColor={surplusColor}
+            />
+            <StatCard
+              label="Bills Paid"
+              value={`${data.bills.paid_count} of ${data.bills.count}`}
+            />
+          </div>
+
+          <div style={styles.tableWrapper}>
+            <table style={styles.table}>
+              <thead>
+                <tr>
+                  <th style={styles.th}>Category</th>
+                  <th style={{ ...styles.th, textAlign: 'right' }}>Amount</th>
+                </tr>
+              </thead>
+              <tbody>
+                <tr style={styles.tr}>
+                  <td style={styles.td}>Income (net)</td>
+                  <td style={{ ...styles.td, ...styles.tdRight, color: '#2a7a2a' }}>
+                    {formatCurrency(data.income.net)}
+                  </td>
+                </tr>
+                <tr style={styles.tr}>
+                  <td style={styles.td}>Bills (planned)</td>
+                  <td style={{ ...styles.td, ...styles.tdRight }}>
+                    -{formatCurrency(data.bills.planned)}
+                  </td>
+                </tr>
+                <tr style={styles.tr}>
+                  <td style={styles.td}>Variable spending</td>
+                  <td style={{ ...styles.td, ...styles.tdRight }}>
+                    -{formatCurrency(data.actuals.total)}
+                  </td>
+                </tr>
+                <tr style={styles.tr}>
+                  <td style={styles.td}>One-time expenses</td>
+                  <td style={{ ...styles.td, ...styles.tdRight }}>
+                    -{formatCurrency(data.one_time_expenses.total)}
+                  </td>
+                </tr>
+                <tr style={styles.trTotal}>
+                  <td style={{ ...styles.td, ...styles.tdBold }}>Surplus / Deficit</td>
+                  <td style={{ ...styles.td, ...styles.tdRight, ...styles.tdBold, color: surplusColor }}>
+                    {formatCurrency(data.summary.surplus_deficit)}
+                  </td>
+                </tr>
+              </tbody>
+            </table>
+          </div>
+        </>
+      )}
+    </div>
+  );
+}
+
+const styles = {
+  container: {
+    maxWidth: '860px',
+    margin: '0 auto',
+  },
+  monthNav: {
+    display: 'flex',
+    alignItems: 'center',
+    gap: '1rem',
+    marginBottom: '1.5rem',
+  },
+  navButton: {
+    padding: '0.3rem 0.75rem',
+    fontSize: '1rem',
+    cursor: 'pointer',
+    border: '1px solid #bbb',
+    borderRadius: '4px',
+    background: '#f5f5f5',
+  },
+  monthLabel: {
+    fontSize: '1.25rem',
+    fontWeight: '600',
+    minWidth: '160px',
+    textAlign: 'center',
+  },
+  errorBanner: {
+    background: '#fde8e8',
+    border: '1px solid #f5a0a0',
+    borderRadius: '4px',
+    padding: '0.75rem 1rem',
+    marginBottom: '1rem',
+    color: '#c0392b',
+  },
+  loadingMsg: {
+    padding: '2rem',
+    color: '#888',
+  },
+  cardRow: {
+    display: 'flex',
+    gap: '1rem',
+    marginBottom: '1rem',
+    flexWrap: 'wrap',
+  },
+  card: {
+    flex: '1 1 160px',
+    border: '1px solid #ddd',
+    borderRadius: '6px',
+    padding: '1rem',
+    background: '#fafafa',
+    minWidth: '140px',
+  },
+  cardLabel: {
+    fontSize: '0.8rem',
+    color: '#666',
+    marginBottom: '0.4rem',
+    fontWeight: '500',
+    textTransform: 'uppercase',
+    letterSpacing: '0.03em',
+  },
+  cardValue: {
+    fontSize: '1.35rem',
+    fontWeight: '700',
+    fontVariantNumeric: 'tabular-nums',
+  },
+  tableWrapper: {
+    marginTop: '1.5rem',
+    border: '1px solid #ddd',
+    borderRadius: '6px',
+    overflow: 'hidden',
+  },
+  table: {
+    width: '100%',
+    borderCollapse: 'collapse',
+    fontSize: '0.95rem',
+  },
+  th: {
+    padding: '0.6rem 1rem',
+    background: '#f0f0f0',
+    fontWeight: '600',
+    color: '#444',
+    borderBottom: '1px solid #ddd',
+    textAlign: 'left',
+  },
+  tr: {
+    borderBottom: '1px solid #eee',
+  },
+  trTotal: {
+    borderTop: '2px solid #ccc',
+    background: '#fafafa',
+  },
+  td: {
+    padding: '0.6rem 1rem',
+    color: '#333',
+  },
+  tdRight: {
+    textAlign: 'right',
+    fontVariantNumeric: 'tabular-nums',
+  },
+  tdBold: {
+    fontWeight: '700',
+    fontSize: '1rem',
+  },
+};
+
 export default MonthlySummary;
diff --git a/server/src/index.js b/server/src/index.js
index 287f469..068e93a 100644
--- a/server/src/index.js
+++ b/server/src/index.js
@@ -8,6 +8,7 @@ const billsRouter = require('./routes/bills');
 const paychecksRouter = require('./routes/paychecks');
 const actualsRouter = require('./routes/actuals');
 const oneTimeExpensesRouter = require('./routes/one-time-expenses');
+const summaryRouter = require('./routes/summary');
 const db = require('./db');
 
 const app = express();
@@ -23,6 +24,7 @@ app.use('/api', billsRouter);
 app.use('/api', paychecksRouter);
 app.use('/api', actualsRouter);
 app.use('/api', oneTimeExpensesRouter);
+app.use('/api', summaryRouter);
 
 // Serve static client files in production
 const clientDist = path.join(__dirname, '../../client/dist');
diff --git a/server/src/routes/summary.js b/server/src/routes/summary.js
new file mode 100644
index 0000000..c72eb11
--- /dev/null
+++ b/server/src/routes/summary.js
@@ -0,0 +1,128 @@
+const express = require('express');
+const router = express.Router();
+const { pool } = require('../db');
+
+const MONTH_NAMES = [
+  'January', 'February', 'March', 'April', 'May', 'June',
+  'July', 'August', 'September', 'October', 'November', 'December',
+];
+
+// GET /api/summary/monthly?year=&month=
+router.get('/summary/monthly', async (req, res) => {
+  const year = parseInt(req.query.year, 10);
+  const month = parseInt(req.query.month, 10);
+
+  if (isNaN(year) || isNaN(month) || month < 1 || month > 12) {
+    return res.status(400).json({ error: 'year and month (1-12) are required query params' });
+  }
+
+  try {
+    // Fetch paychecks for this month
+    const pcResult = await pool.query(
+      `SELECT id, gross, net FROM paychecks
+       WHERE period_year = $1 AND period_month = $2`,
+      [year, month]
+    );
+
+    // If no paychecks exist, return zeros
+    if (pcResult.rows.length === 0) {
+      return res.json({
+        year,
+        month,
+        month_name: MONTH_NAMES[month - 1],
+        income: { gross: 0, net: 0 },
+        bills: { planned: 0, paid: 0, unpaid: 0, count: 0, paid_count: 0 },
+        actuals: { total: 0, count: 0 },
+        one_time_expenses: { total: 0, count: 0 },
+        summary: { total_spending: 0, surplus_deficit: 0 },
+      });
+    }
+
+    const paycheckIds = pcResult.rows.map(r => r.id);
+
+    // Income totals
+    const incomeGross = pcResult.rows.reduce((sum, r) => sum + (parseFloat(r.gross) || 0), 0);
+    const incomeNet = pcResult.rows.reduce((sum, r) => sum + (parseFloat(r.net) || 0), 0);
+
+    // Bills aggregates
+    const billsResult = await pool.query(
+      `SELECT
+         COUNT(*)::int AS count,
+         COALESCE(SUM(CASE WHEN pb.amount_override IS NOT NULL THEN pb.amount_override ELSE b.amount END), 0) AS planned,
+         COALESCE(SUM(CASE WHEN pb.paid THEN CASE WHEN pb.amount_override IS NOT NULL THEN pb.amount_override ELSE b.amount END ELSE 0 END), 0) AS paid_amount,
+         COUNT(*) FILTER (WHERE pb.paid)::int AS paid_count
+       FROM paycheck_bills pb
+       JOIN bills b ON b.id = pb.bill_id
+       WHERE pb.paycheck_id = ANY($1)`,
+      [paycheckIds]
+    );
+
+    const billsRow = billsResult.rows[0];
+    const billsPlanned = parseFloat(billsRow.planned) || 0;
+    const billsPaid = parseFloat(billsRow.paid_amount) || 0;
+    const billsUnpaid = billsPlanned - billsPaid;
+    const billsCount = billsRow.count || 0;
+    const billsPaidCount = billsRow.paid_count || 0;
+
+    // Actuals aggregates
+    const actualsResult = await pool.query(
+      `SELECT COUNT(*)::int AS count, COALESCE(SUM(amount), 0) AS total
+       FROM actuals
+       WHERE paycheck_id = ANY($1)`,
+      [paycheckIds]
+    );
+
+    const actualsRow = actualsResult.rows[0];
+    const actualsTotal = parseFloat(actualsRow.total) || 0;
+    const actualsCount = actualsRow.count || 0;
+
+    // One-time expenses aggregates
+    const oteResult = await pool.query(
+      `SELECT COUNT(*)::int AS count, COALESCE(SUM(amount), 0) AS total
+       FROM one_time_expenses
+       WHERE paycheck_id = ANY($1)`,
+      [paycheckIds]
+    );
+
+    const oteRow = oteResult.rows[0];
+    const oteTotal = parseFloat(oteRow.total) || 0;
+    const oteCount = oteRow.count || 0;
+
+    const totalSpending = billsPlanned + actualsTotal + oteTotal;
+    const surplusDeficit = incomeNet - totalSpending;
+
+    res.json({
+      year,
+      month,
+      month_name: MONTH_NAMES[month - 1],
+      income: {
+        gross: parseFloat(incomeGross.toFixed(2)),
+        net: parseFloat(incomeNet.toFixed(2)),
+      },
+      bills: {
+        planned: parseFloat(billsPlanned.toFixed(2)),
+        paid: parseFloat(billsPaid.toFixed(2)),
+        unpaid: parseFloat(billsUnpaid.toFixed(2)),
+        count: billsCount,
+        paid_count: billsPaidCount,
+      },
+      actuals: {
+        total: parseFloat(actualsTotal.toFixed(2)),
+        count: actualsCount,
+      },
+      one_time_expenses: {
+        total: parseFloat(oteTotal.toFixed(2)),
+        count: oteCount,
+      },
+      summary: {
+        total_spending: parseFloat(totalSpending.toFixed(2)),
+        surplus_deficit: parseFloat(surplusDeficit.toFixed(2)),
+      },
+    });
+  } catch (err) {
+    console.error('GET /api/summary/monthly error:', err);
+    res.status(500).json({ error: 'Failed to fetch monthly summary' });
+  }
+});
+
+module.exports = router;