Abstract. In this paper, an integrated approach for the modeling and the validation through simulation of multi-agent systems is proposed. The approach centers on the instantiation of a software development process which specifically includes a simulation phase which makes it possible the validation of a multiagent system before its actual deployment and execution. The approach uses the Gaia methodology for the analysis and the design, the Agent UML and the Distilled StateCharts for the detailed design, the MAO Framework for the neutral-platform implementation of software agents, and a Java-based discreteevent simulation framework for the simulation. The proposed approach is exemplified by defining and simulating a multi-agent system concerning with an agent-mediated consumer-driven e-Marketplace which offers mobile agentbased services for searching and buying goods.