1. INTRODUCTION Recently the agent-oriented approach [1] has been recog- nized as very suitable for the development of complex soft- ware systems. In particular:  the agent-oriented decomposition is an effective way of partitioning the problem space;  the key abstractions of the agent-oriented mindset (agents, interactions, and organizations) are a natural means of modelling;  the agent-oriented paradigm for modelling and managing organizational relationships is appropriate for dealing with existing dependencies and interactions. However, the development of complex software systems by using the agent-oriented approach requires suitable agent- oriented modelling techniques and methodologies which provide explicit support for the key abstractions of the agent paradigm. Several methodologies supporting analysis, design and implementation of Multi-Agent Systems (MAS) have been proposed in the context of Agent Oriented Software Engineering (AOSE) [2]. Some of the emerging method- ologies are Gaia [3], MaSE [4], Prometheus [5], Tropos [6], Message [7], Passi [8], and Adelfe [9]. These notable efforts basically provide a top-down iterative approach for the modelling and the development of agent-based sys- tems. Although, from the software development life-cycle point of view, such methodologies fully cover require- ments capture, architectural design and detailed design, the implementation phase is currently not well supported [10]. In fact, it is difficult to map design models onto different target agent platforms since a seamless translation process is usually not provided. Furthermore, none of these methodologies supports validation through simulation of the MAS under development before its actual deployment and execution. Validation through simulation or through formal methods can demonstrate that a MAS correctly behaves according to its specifications. In particular, dis- crete-event simulators are highly required for evaluating how complex MAS work on scales much larger than those vol 20 no 4 july 2005 259 Comput Syst Sci & Eng (2005) 4: 259–271 © 2005 CRL Publishing Ltd An integrated approach for the development and validation of multi-agent systems Giancarlo Fortino, Alfredo Garro and Wilma Russo D.E.I.S.,Università della Calabria, Via P. Bucci, cubo 41C, 87036 Rende (CS), Italy. Email: {g.fortino,a.garro,w.russo}@unical.it International Journal of Computer Systems Science & Engineering In this paper, an integrated approach for the development and validation through simulation of multi-agent systems is proposed and exemplified through a case study concerning with an agent-based consumer-driven e-Marketplace. The approach centres on the instantiation of a software development process by using the method engineering paradigm. As a distinctive feature this process specifically includes a simulation phase which makes it possible the validation of a multi-agent system under development before its actual deployment and execution. In particular, the integrated 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 discrete-event simulation framework for the simulation. Keywords: multi-agent systems, method engineering, event-driven simulation, Distilled StateCharts