diff --git a/lib/src/invoice/invoice.dart b/lib/src/invoice/invoice.dart
new file mode 100644
index 0000000..c7fb85e
--- /dev/null
+++ b/lib/src/invoice/invoice.dart
@@ -0,0 +1,175 @@
+import 'money.dart';
+
+class VatRate {
+  final String label;
+  final double rate;
+  const VatRate(this.label, this.rate);
+
+  static const standard = VatRate('23%', 0.23);
+  static const reduced8 = VatRate('8%', 0.08);
+  static const reduced5 = VatRate('5%', 0.05);
+  static const zero = VatRate('0%', 0.0);
+  static const exempt = VatRate('zw', 0.0);
+
+  static VatRate fromPercent(num percent) => VatRate('$percent%', percent / 100);
+
+  @override
+  bool operator ==(Object other) => other is VatRate && other.label == label && other.rate == rate;
+
+  @override
+  int get hashCode => Object.hash(label, rate);
+}
+
+class Party {
+  final String name;
+  final String? address;
+  final String? taxId;
+  final String? email;
+
+  const Party({required this.name, this.address, this.taxId, this.email});
+}
+
+class InvoiceLine {
+  final String description;
+  final num quantity;
+  final Money unitPrice;
+  final double discountRate;
+  final VatRate vat;
+  final String unit;
+
+  const InvoiceLine({
+    required this.description,
+    required this.quantity,
+    required this.unitPrice,
+    required this.vat,
+    this.discountRate = 0.0,
+    this.unit = 'pcs',
+  });
+
+  Money get grossBeforeDiscount => unitPrice.scaled(quantity);
+
+  Money get discountAmount => grossBeforeDiscount.scaled(discountRate);
+
+  Money get net => grossBeforeDiscount - discountAmount;
+
+  Money get vatAmount => net.scaled(vat.rate);
+
+  Money get total => net + vatAmount;
+}
+
+class VatSummaryRow {
+  final VatRate rate;
+  final Money net;
+  final Money vat;
+
+  const VatSummaryRow(this.rate, this.net, this.vat);
+
+  Money get gross => net + vat;
+}
+
+class Invoice {
+  final String number;
+  final DateTime issueDate;
+  final DateTime? dueDate;
+  final Party seller;
+  final Party buyer;
+  final Currency currency;
+  final List<InvoiceLine> lines;
+  final String? notes;
+  final String? paymentTerms;
+  final Currency? reportingCurrency;
+  final double? reportingRate;
+
+  const Invoice({
+    required this.number,
+    required this.issueDate,
+    required this.seller,
+    required this.buyer,
+    required this.currency,
+    required this.lines,
+    this.dueDate,
+    this.notes,
+    this.paymentTerms,
+    this.reportingCurrency,
+    this.reportingRate,
+  });
+
+  List<VatSummaryRow> vatSummary() {
+    final order = <String>[];
+    final rates = <String, VatRate>{};
+    final nets = <String, Money>{};
+    for (final line in lines) {
+      final key = line.vat.label;
+      if (!nets.containsKey(key)) {
+        order.add(key);
+        rates[key] = line.vat;
+        nets[key] = Money.zero(currency);
+      }
+      nets[key] = nets[key]! + line.net;
+    }
+    return [
+      for (final key in order)
+        VatSummaryRow(rates[key]!, nets[key]!, nets[key]!.scaled(rates[key]!.rate)),
+    ];
+  }
+
+  Money get totalNet {
+    var sum = Money.zero(currency);
+    for (final row in vatSummary()) {
+      sum += row.net;
+    }
+    return sum;
+  }
+
+  Money get totalVat {
+    var sum = Money.zero(currency);
+    for (final row in vatSummary()) {
+      sum += row.vat;
+    }
+    return sum;
+  }
+
+  Money get totalGross => totalNet + totalVat;
+
+  Money? get reportingTotalGross {
+    final target = reportingCurrency;
+    final rate = reportingRate;
+    if (target == null || rate == null) return null;
+    return totalGross.convertedTo(target, rate);
+  }
+
+  Money? get reportingTotalVat {
+    final target = reportingCurrency;
+    final rate = reportingRate;
+    if (target == null || rate == null) return null;
+    return totalVat.convertedTo(target, rate);
+  }
+}
+
+class InvoiceSequence {
+  final String pattern;
+  int counter;
+
+  InvoiceSequence(this.pattern, {this.counter = 1});
+
+  String format(DateTime date, int seq) {
+    return pattern.replaceAllMapped(RegExp(r'\{(\w+)(?::(\d+))?\}'), (match) {
+      final field = match.group(1)!;
+      final width = match.group(2) == null ? 0 : int.parse(match.group(2)!);
+      final value = switch (field) {
+        'seq' => seq,
+        'year' => date.year,
+        'month' => date.month,
+        'day' => date.day,
+        _ => throw ArgumentError('Unknown numbering field: $field'),
+      };
+      return value.toString().padLeft(width, '0');
+    });
+  }
+
+  String next(DateTime date) {
+    final formatted = format(date, counter);
+    counter++;
+    return formatted;
+  }
+}