Agent based technologies in the sense of distributed computing becomes increasingly relevant in academic and industrial research and development. Our multi agent system CASA focuses on the specification of complex plans that describe the behavior of agents. The design of CASA is based on concepts from concurrent logic programming that were used to extend the well known BDI agent approach AgentSpeak(L) [2]. Rao’s AgentSpeak(L) is a specification language similar to horn clauses and can be viewed as an abstraction of an implemented BDI system. Our work is based on the assumption that AgentSpeak(L) demonstrates a successful reengineering approach of an implemented multi agent system that is now given a formal specification. By extending this specification with new features for complex plans it was possible to derive an efficient implementation that supports these new features.