What is the Singleton pattern and when should you use it?

Singleton ensures a class has only one instance and provides global access to it. Implementation: private constructor, static method returning the single instance. Use cases: configuration managers, connection pools, loggers. Criticisms: global state, makes testing difficult, hides dependencies. Modern alternatives: dependency injection for managing single instances. If using Singleton, consider thread-safety (double-checked locking, static initialization). Use sparingly and prefer dependency injection.