diff --git a/backend/src/modules/banking/paymentService.ts b/backend/src/modules/banking/paymentService.ts
new file mode 100644
index 0000000..17c3f4f
--- /dev/null
+++ b/backend/src/modules/banking/paymentService.ts
@@ -0,0 +1,182 @@
+import { Prisma, PrismaClient, PaymentStatus } from '@prisma/client';
+import { IncomingBankWebhookPayload, RentTrafficLight } from './types';
+
+type ContractWithPayments = Prisma.ContractGetPayload<{
+  include: { room: true; user: true; payments: true };
+}>;
+type LandlordUser = Prisma.UserGetPayload<{}>;
+import { findMatchingPayment } from './matching';
+import { sendPushNotification } from '../../services/notificationService';
+
+export class PaymentService {
+  constructor(private prisma: PrismaClient) {}
+
+  /**
+   * Verarbeitet eine eingehende Banktransaktion: Matching, Statuswechsel,
+   * Speicherung der bank_transaction_id (idempotent bei erneutem Webhook-Delivery).
+   */
+  async processIncomingTransaction(tx: IncomingBankWebhookPayload) {
+    const alreadyProcessed = await this.prisma.payment.findUnique({
+      where: { bankTransactionId: tx.transactionId },
+    });
+    if (alreadyProcessed) {
+      return { status: 'already_processed' as const, paymentId: alreadyProcessed.id };
+    }
+
+    const match = await findMatchingPayment(this.prisma, tx);
+
+    if (match.confidence === 'UNCERTAIN' || !match.paymentId) {
+      // Keine automatische Zuordnung: Transaktion wird als "unzugeordnet" geloggt,
+      // damit der Vermieter sie im Cockpit manuell matchen kann (siehe Modell
+      // `UnmatchedTransactionLog` in prisma/schema.prisma).
+      await this.prisma.unmatchedTransactionLog.create({
+        data: {
+          bankTransactionId: tx.transactionId,
+          amount: tx.amount,
+          remittanceInfo: tx.remittanceInfo,
+          reason: match.reason,
+        },
+      });
+
+      await this.notifyLandlord(
+        'RENT_MATCH_UNCERTAIN',
+        'Zahlung erfordert manuelle Prüfung',
+        `Eingang über ${tx.amount.toFixed(2)} € konnte nicht automatisch zugeordnet werden: ${match.reason}`,
+      );
+
+      return { status: 'uncertain' as const, reason: match.reason };
+    }
+
+    const payment = await this.prisma.payment.update({
+      where: { id: match.paymentId },
+      data: {
+        status: PaymentStatus.PAID,
+        paidAt: new Date(tx.bookingDate),
+        bankTransactionId: tx.transactionId,
+        bankReferenceText: tx.remittanceInfo,
+        matchedAutomatically: match.confidence !== 'MEDIUM' ? true : true,
+      },
+      include: { contract: { include: { user: true, room: true } } },
+    });
+
+    if (match.confidence === 'MEDIUM') {
+      // Trotzdem automatisch gebucht, aber Vermieter zur Kontrolle informiert.
+      await this.notifyLandlord(
+        'RENT_MATCHED_MEDIUM_CONFIDENCE',
+        'Miete gebucht (bitte kurz prüfen)',
+        `${payment.contract.user.fullName} / ${payment.contract.room.roomNumber}: ${tx.amount.toFixed(2)} € automatisch zugeordnet (mittlere Konfidenz).`,
+      );
+    }
+
+    return { status: 'matched' as const, paymentId: payment.id, confidence: match.confidence };
+  }
+
+  /**
+   * Berechnet die Ampel-Status je aktivem Vertrag für das Vermieter-Cockpit.
+   * GRÜN = bezahlt, GELB = fällig aber Frist (3. Werktag) nicht erreicht, ROT = überfällig.
+   */
+  async getRentTrafficLights(referenceDate: Date = new Date()): Promise<RentTrafficLight[]> {
+    const activeContracts = await this.prisma.contract.findMany({
+      where: { isActive: true },
+      include: {
+        room: true,
+        user: true,
+        payments: {
+          where: {
+            dueDate: {
+              gte: startOfMonth(referenceDate),
+              lte: endOfMonth(referenceDate),
+            },
+          },
+        },
+      },
+    });
+
+    return activeContracts.map((c: ContractWithPayments) => {
+      const currentPayment = c.payments[0];
+      const dueThreshold = thirdBusinessDay(referenceDate);
+      const paid = currentPayment?.status === PaymentStatus.PAID;
+
+      let status: RentTrafficLight['status'] = 'YELLOW';
+      if (paid) status = 'GREEN';
+      else if (referenceDate > dueThreshold) status = 'RED';
+
+      return {
+        contractId: c.id,
+        roomNumber: c.room.roomNumber,
+        tenantName: c.user.fullName,
+        status,
+        dueDate: currentPayment?.dueDate.toISOString() ?? '',
+        amountDue: Number(currentPayment?.amount ?? c.totalWarmRent),
+        amountPaid: paid ? Number(currentPayment.amount) : 0,
+      };
+    });
+  }
+
+  /**
+   * Täglicher Cron-Job: findet alle ROT-Fälle und verschickt (max. 1x) eine
+   * Mahn-Notification, sofern noch keine für diesen Zahlungslauf gesendet wurde.
+   */
+  async runDailyDunningCheck(referenceDate: Date = new Date()) {
+    const overdue = await this.prisma.payment.findMany({
+      where: {
+        status: { in: [PaymentStatus.PENDING, PaymentStatus.OVERDUE] },
+        dueDate: { lt: thirdBusinessDay(referenceDate) },
+        dunningSentAt: null,
+      },
+      include: { contract: { include: { user: true, room: true } } },
+    });
+
+    for (const payment of overdue) {
+      await this.prisma.payment.update({
+        where: { id: payment.id },
+        data: { status: PaymentStatus.OVERDUE, dunningSentAt: new Date() },
+      });
+
+      await sendPushNotification({
+        userId: payment.contract.userId,
+        type: 'RENT_OVERDUE',
+        title: 'Miete überfällig',
+        body: `Deine Miete für ${payment.contract.room.roomNumber} (${Number(payment.amount).toFixed(2)} €) ist überfällig. Bitte zeitnah überweisen.`,
+      });
+
+      await this.notifyLandlord(
+        'RENT_OVERDUE_LANDLORD',
+        `Miete überfällig: ${payment.contract.room.roomNumber}`,
+        `${payment.contract.user.fullName} hat die Miete (${Number(payment.amount).toFixed(2)} €) noch nicht bezahlt.`,
+      );
+    }
+
+    return { dunningsSent: overdue.length };
+  }
+
+  private async notifyLandlord(type: string, title: string, body: string) {
+    const landlords = await this.prisma.user.findMany({ where: { role: 'LANDLORD' } });
+    await Promise.all(
+      landlords.map((l: LandlordUser) => sendPushNotification({ userId: l.id, type, title, body })),
+    );
+  }
+}
+
+// --- Datumshilfen -----------------------------------------------------------
+
+function startOfMonth(d: Date): Date {
+  return new Date(d.getFullYear(), d.getMonth(), 1);
+}
+
+function endOfMonth(d: Date): Date {
+  return new Date(d.getFullYear(), d.getMonth() + 1, 0, 23, 59, 59);
+}
+
+/** Berechnet den 3. Werktag (Mo-Fr, ohne Feiertagsberücksichtigung) des Monats von `d`. */
+function thirdBusinessDay(d: Date): Date {
+  let count = 0;
+  const cursor = startOfMonth(d);
+  while (count < 3) {
+    const day = cursor.getDay();
+    if (day !== 0 && day !== 6) count++;
+    if (count < 3) cursor.setDate(cursor.getDate() + 1);
+  }
+  cursor.setHours(23, 59, 59, 999);
+  return cursor;
+}