Patterns systems are becoming more and more numerous. They offer product patterns or process patterns of varied range and cover (analysis design or implementation patterns, and general, domain or enterprise patterns). New application development environments have been developed together with these pattern-oriented approaches. These tools address two kinds of actors: patterns engineers who specify patterns systems, and applications engineers who use these systems to specify information systems. Most of the existing development environments are made for applications engineers; they offer few functionalities allowing definition and organization of patterns systems. This paper presents AGAP, a development environment for defining and using patterns, which distinguishes pattern formalisms from patterns systems. Not only does AGAP address applications engineers, but it also allows patterns engineers to define patterns systems. The same formalisms or items of existing formalisms may either be used in order to facilitate the engineering of patterns systems or to increase the level of reuse.