Implement and explain the Singleton design pattern.

Singleton ensures a class has only one instance with global access. Implementation: private constructor, private static instance, public static getInstance() method. Thread-safe options: synchronized getInstance (slow), eager initialization (simple), double-checked locking (volatile + sync), enum singleton (Java, best practice), static holder class (lazy, thread-safe). Use cases: logging, configuration, connection pools. Criticisms: global state, testing difficulty, hidden dependencies. Consider dependency injection as alternative. Ensure: private constructor, static instance, controlled access.