Abstract: This paper presents a new approach of software process evolution. This new approach deals with the evolution of both software process and their models in Process Centered Software Engineering Environments (PSEE's). These two types of evolution are complementary. The first one allows acting directly on the process during its execution. By appropriate update operations, it gives to the user the possibility to perfect his software process through simulation and to define new software processes dynamically. The second one allows evolving software process models to satisfy new software engineering requirements (new software component types, new tool types, new agent types, etc). We provide an innovative approach which supports different evolution strategies.