Multi-agent systems seem to provide a good basis to build complex agent based simulation systems, but this paper points out some of the drawbacks of classical “agent centered” multi- agent systems and propose a solution to these problems by using “organization centered multi-agent system”, or OCMAS for short. We propose a set of general principles from which true OCMAS may be designed. One of these principles is not to assume anything about the cognitive capabilities of agents. In order to show how OCMAS models may be designed, we show how a very concise and minimal OCMAS model called AGR, for Agent/Group/Role, may be used to build true OCMAS systems. We propose a set of notations and a methodological framework to help the designer to build MAS using AGR.