In recent years, agent-based systems have received considerable attention in both academia and industry. The agent oriented paradigm can be considered a natural extension to the object oriented (OO) paradigm. In spite of the fact that there are many OO analysis and design methods, there is very little work reported on design and analysis of agent-based systems. Agents differ from objects in many issues which requires special modeling elements. In this paper, we propose a software engineering process for agent-based systems based on existing object-oriented software engineering concepts. We extend the Unified Modeling Language (UML) to model agents (static and mobile).