What are metaclasses in Python?

Metaclasses are 'classes of classes' - they define how classes behave. Class is instance of metaclass (usually 'type'). Use: class MyClass(metaclass=MyMeta). Metaclass __new__ creates class, __init__ initializes it. Use cases: registering classes automatically, enforcing API contracts, adding methods/attributes, ORMs (Django models), abstract base classes. Example: auto-register all subclasses of a base class. Most Python code doesn't need metaclasses - use decorators, class decorators, or __init_subclass__ when possible. 'Metaclasses are deeper magic than 99% of users need.'