diff --git a/README.md b/README.md
index cab3c08..76507d5 100644
--- a/README.md
+++ b/README.md
@@ -1,3 +1,4 @@
+[![Review Assignment Due Date](https://classroom.github.com/assets/deadline-readme-button-22041afd0340ce965d47ae6ef1cefeee28c7c493a6346c4f15d667ab976d596c.svg)](https://classroom.github.com/a/W3nV4mdD)
 # Banking management
 
 ## Overview
diff --git a/src/is-equal.ts b/src/is-equal.ts
new file mode 100644
index 0000000..78aebbb
--- /dev/null
+++ b/src/is-equal.ts
@@ -0,0 +1,3 @@
+export default function isEqual(a: unknown, b: unknown): boolean {
+  return JSON.stringify(a) === JSON.stringify(b);
+}
diff --git a/src/models/BankAccount.ts b/src/models/BankAccount.ts
new file mode 100644
index 0000000..c94aa11
--- /dev/null
+++ b/src/models/BankAccount.ts
@@ -0,0 +1 @@
+export { default } from '@/models/bank-account';
diff --git a/src/models/bank-account.ts b/src/models/bank-account.ts
new file mode 100644
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--- /dev/null
+++ b/src/models/bank-account.ts
@@ -0,0 +1,76 @@
+import { v4 as uuidv4 } from 'uuid';
+import type { BankAccountId, BankId } from '@/types/Common';
+
+interface BankAccountOptions {
+  isNegativeAllowed?: boolean;
+}
+
+export default class BankAccount {
+  private readonly id: BankAccountId;
+  private readonly bankId: BankId;
+  private balance: number;
+  private readonly isNegativeAllowed: boolean;
+
+  private constructor(
+    id: BankAccountId,
+    bankId: BankId,
+    initialBalance: number,
+    options?: BankAccountOptions
+  ) {
+    this.id = id;
+    this.bankId = bankId;
+    this.balance = initialBalance;
+    this.isNegativeAllowed = options?.isNegativeAllowed ?? false;
+  }
+
+  static create(bankId: BankId, initialBalance = 0, options?: BankAccountOptions): BankAccount {
+    return new BankAccount(uuidv4(), bankId, initialBalance, options);
+  }
+
+  getId(): BankAccountId {
+    return this.id;
+  }
+
+  getBankId(): BankId {
+    return this.bankId;
+  }
+
+  getBalance(): number {
+    return this.balance;
+  }
+
+  canDebit(amount: number): boolean {
+    if (amount < 0) {
+      return false;
+    }
+    if (this.isNegativeAllowed) {
+      return true;
+    }
+    return this.balance >= amount;
+  }
+
+  credit(amount: number): void {
+    if (amount <= 0) {
+      throw new Error('Amount should be greater than zero');
+    }
+    this.balance += amount;
+  }
+
+  debit(amount: number): void {
+    if (amount <= 0) {
+      throw new Error('Amount should be greater than zero');
+    }
+    if (!this.canDebit(amount)) {
+      throw new Error('Insufficient funds');
+    }
+    this.balance -= amount;
+  }
+
+  deposit(amount: number): void {
+    this.credit(amount);
+  }
+
+  withdraw(amount: number): void {
+    this.debit(amount);
+  }
+}
diff --git a/src/models/bank.ts b/src/models/bank.ts
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index 0000000..2aa563e
--- /dev/null
+++ b/src/models/bank.ts
@@ -0,0 +1,55 @@
+import { v4 as uuidv4 } from 'uuid';
+import BankAccount from '@/models/bank-account';
+import type { BankAccountId, BankId, UserId } from '@/types/Common';
+import GlobalRegistry from '@/services/GlobalRegistry';
+import TransactionService from '@/services/TransactionService';
+
+interface BankOptions {
+  isNegativeAllowed?: boolean;
+}
+
+export default class Bank {
+  private readonly id: BankId;
+  private readonly isNegativeAllowed: boolean;
+  private readonly accountIds: Set<BankAccountId>;
+
+  private constructor(id: BankId, options?: BankOptions) {
+    this.id = id;
+    this.isNegativeAllowed = options?.isNegativeAllowed ?? false;
+    this.accountIds = new Set<BankAccountId>();
+  }
+
+  static create(options?: BankOptions): Bank {
+    const bank = new Bank(uuidv4(), options);
+    GlobalRegistry.registerBank(bank);
+    return bank;
+  }
+
+  getId(): BankId {
+    return this.id;
+  }
+
+  createAccount(initialBalance = 0): BankAccount {
+    const account = BankAccount.create(this.id, initialBalance, {
+      isNegativeAllowed: this.isNegativeAllowed
+    });
+    this.accountIds.add(account.getId());
+    GlobalRegistry.registerAccount(account);
+    return account;
+  }
+
+  getAccount(accountId: BankAccountId): BankAccount {
+    if (!this.accountIds.has(accountId)) {
+      throw new Error('Bank account not found');
+    }
+    return GlobalRegistry.getAccount(accountId);
+  }
+
+  send(fromUserId: UserId, toUserId: UserId, amount: number, toBankId?: BankId): void {
+    TransactionService.transfer(this.id, fromUserId, toUserId, amount, toBankId);
+  }
+
+  transfer(fromUserId: UserId, toUserId: UserId, amount: number, toBankId?: BankId): void {
+    this.send(fromUserId, toUserId, amount, toBankId);
+  }
+}
diff --git a/src/models/user.ts b/src/models/user.ts
new file mode 100644
index 0000000..147868a
--- /dev/null
+++ b/src/models/user.ts
@@ -0,0 +1,37 @@
+import { v4 as uuidv4 } from 'uuid';
+import type { BankAccountId, UserId } from '@/types/Common';
+import GlobalRegistry from '@/services/GlobalRegistry';
+
+export default class User {
+  private readonly id: UserId;
+  private readonly name: string;
+  private readonly accountIds: BankAccountId[];
+
+  private constructor(id: UserId, name: string, accountIds: BankAccountId[]) {
+    this.id = id;
+    this.name = name;
+    this.accountIds = accountIds;
+  }
+
+  static create(name: string, accountIds: BankAccountId[]): User {
+    const user = new User(uuidv4(), name, [...accountIds]);
+    GlobalRegistry.registerUser(user);
+    return user;
+  }
+
+  getId(): UserId {
+    return this.id;
+  }
+
+  getName(): string {
+    return this.name;
+  }
+
+  getAccountIds(): BankAccountId[] {
+    return [...this.accountIds];
+  }
+
+  getAccounts(): BankAccountId[] {
+    return this.getAccountIds();
+  }
+}
diff --git a/src/services/GlobalRegistry.ts b/src/services/GlobalRegistry.ts
new file mode 100644
index 0000000..461a3ed
--- /dev/null
+++ b/src/services/GlobalRegistry.ts
@@ -0,0 +1,52 @@
+import type Bank from '@/models/bank';
+import type BankAccount from '@/models/bank-account';
+import type User from '@/models/user';
+import type { BankAccountId, BankId, UserId } from '@/types/Common';
+
+export default class GlobalRegistry {
+  private static banks = new Map<BankId, Bank>();
+  private static users = new Map<UserId, User>();
+  private static accounts = new Map<BankAccountId, BankAccount>();
+
+  static registerBank(bank: Bank): void {
+    this.banks.set(bank.getId(), bank);
+  }
+
+  static registerUser(user: User): void {
+    this.users.set(user.getId(), user);
+  }
+
+  static registerAccount(account: BankAccount): void {
+    this.accounts.set(account.getId(), account);
+  }
+
+  static getBank(bankId: BankId): Bank {
+    const bank = this.banks.get(bankId);
+    if (!bank) {
+      throw new Error('Bank not found');
+    }
+    return bank;
+  }
+
+  static getUser(userId: UserId): User {
+    const user = this.users.get(userId);
+    if (!user) {
+      throw new Error('User not found');
+    }
+    return user;
+  }
+
+  static getAccount(accountId: BankAccountId): BankAccount {
+    const account = this.accounts.get(accountId);
+    if (!account) {
+      throw new Error('Bank account not found');
+    }
+    return account;
+  }
+
+  static clear(): void {
+    this.banks.clear();
+    this.users.clear();
+    this.accounts.clear();
+  }
+}
diff --git a/src/services/TransactionService.ts b/src/services/TransactionService.ts
new file mode 100644
index 0000000..ccdf93e
--- /dev/null
+++ b/src/services/TransactionService.ts
@@ -0,0 +1,119 @@
+import type BankAccount from '@/models/bank-account';
+import type { BankId, UserId } from '@/types/Common';
+import GlobalRegistry from '@/services/GlobalRegistry';
+
+export default class TransactionService {
+  static transfer(
+    fromBankId: BankId,
+    fromUserId: UserId,
+    toUserId: UserId,
+    amount: number,
+    toBankId?: BankId
+  ): void {
+    if (amount <= 0) {
+      throw new Error('Amount should be greater than zero');
+    }
+
+    const sourceBankId = fromBankId;
+    const targetBankId = toBankId ?? fromBankId;
+    const isSelfTransfer = fromUserId === toUserId;
+
+    const fromUser = GlobalRegistry.getUser(fromUserId);
+    const toUser = GlobalRegistry.getUser(toUserId);
+
+    // Validate banks exist before continuing transfer.
+    GlobalRegistry.getBank(sourceBankId);
+    GlobalRegistry.getBank(targetBankId);
+
+    const sourceAccounts = this.pickSourceAccounts(fromUser.getAccountIds(), sourceBankId);
+    const targetAccount = this.pickTargetAccount(
+      toUser.getAccountIds(),
+      targetBankId,
+      isSelfTransfer ? sourceAccounts[0]?.getId() : undefined
+    );
+
+    this.debitAcrossAccounts(sourceAccounts, amount);
+    targetAccount.credit(amount);
+  }
+
+  static send(
+    fromBankId: BankId,
+    fromUserId: UserId,
+    toUserId: UserId,
+    amount: number,
+    toBankId?: BankId
+  ): void {
+    this.transfer(fromBankId, fromUserId, toUserId, amount, toBankId);
+  }
+
+  private static pickSourceAccounts(accountIds: string[], sourceBankId: BankId): BankAccount[] {
+    const sourceAccounts = accountIds
+      .map((accountId) => GlobalRegistry.getAccount(accountId))
+      .filter((account) => account.getBankId() === sourceBankId);
+
+    if (sourceAccounts.length === 0) {
+      throw new Error('Insufficient funds');
+    }
+
+    return sourceAccounts;
+  }
+
+  private static debitAcrossAccounts(sourceAccounts: BankAccount[], amount: number): void {
+    let remaining = amount;
+
+    for (const account of sourceAccounts) {
+      if (remaining <= 0) {
+        break;
+      }
+
+      if (account.canDebit(remaining)) {
+        remaining = 0;
+        continue;
+      }
+
+      const available = account.getBalance();
+      if (available > 0) {
+        remaining -= Math.min(available, remaining);
+      }
+    }
+
+    if (remaining > 0) {
+      throw new Error('Insufficient funds');
+    }
+
+    remaining = amount;
+    for (const account of sourceAccounts) {
+      if (remaining <= 0) {
+        break;
+      }
+
+      if (account.canDebit(remaining)) {
+        account.debit(remaining);
+        remaining = 0;
+        break;
+      }
+
+      const available = account.getBalance();
+      if (available > 0) {
+        const debitAmount = Math.min(available, remaining);
+        account.debit(debitAmount);
+        remaining -= debitAmount;
+      }
+    }
+  }
+
+  private static pickTargetAccount(
+    accountIds: string[],
+    targetBankId: BankId,
+    excludedAccountId?: string
+  ): BankAccount {
+    for (const accountId of accountIds) {
+      const account = GlobalRegistry.getAccount(accountId);
+      if (account.getBankId() === targetBankId && account.getId() !== excludedAccountId) {
+        return account;
+      }
+    }
+
+    throw new Error('Target account not found');
+  }
+}
diff --git a/src/services/global-registry.ts b/src/services/global-registry.ts
new file mode 100644
index 0000000..7f9b813
--- /dev/null
+++ b/src/services/global-registry.ts
@@ -0,0 +1 @@
+export { default } from '@/services/GlobalRegistry';
diff --git a/src/services/transaction-service.ts b/src/services/transaction-service.ts
new file mode 100644
index 0000000..11a9879
--- /dev/null
+++ b/src/services/transaction-service.ts
@@ -0,0 +1 @@
+export { default } from '@/services/TransactionService';
diff --git a/src/types/Common.ts b/src/types/Common.ts
new file mode 100644
index 0000000..6d2d15c
--- /dev/null
+++ b/src/types/Common.ts
@@ -0,0 +1,3 @@
+export type BankId = string;
+export type BankAccountId = string;
+export type UserId = string;