Domain-Driven Design is a software development methodology that tackles complexity by aligning software architecture with business domains through collaborative modeling. Introduced by Eric Evans in 2003, DDD provides both strategic patterns (bounded contexts, context mapping, ubiquitous language) for organizing large systems and tactical patterns (entities, aggregates, value objects, repositories) for implementing rich domain models. The core insight: complex business logic belongs in the domain layer, encapsulated within clearly defined boundaries that reflect how domain experts naturally think about and describe their work — making software not just functional, but maintainable and evolvable as business understanding deepens over time.