The chapter presents data oriented and process oriented models of legacy systems. It discusses the details of systems development and evolution models mainly aiming at an ongoing reengineering of legacy systems. It proposes few strategies for reengineering of both data oriented model and process oriented models. The legacy systems often miss automatic interfaces to external systems, so the chapter presents a strategy focusing on automatic update of data of the system. Likewise, the chapter also presents a strategy for process reengineering in order to integrate external systems. Finally, a legacy system is envisioned as a comprehensive mix of both data and process oriented, while proposing a gradual ongoing reengineering of both data structures and process methods.