Domain-driven design is an approach to developing software thats connected to an evolving model of core business concepts. Onion architecture is built on this concept, with layers connected through interfaces and a focus on keeping external dependencies as far outward as possible. This increases code quality, reduces complexity, and enables evolutionary enterprise systems. It provides a flexible, sustainable, and portable architecture, with layers that are not tightly coupled and have a separation of concerns. With better maintainability, overall code testability, and the ability to easily swap out frameworks or technologies, Onion Architecture is a great approach for software development.