Mobile agents seem an interesting solution for the design and deployment of Web services and applications in the Internet scenario. However, mobility complicates the design of applications and calls for new approaches to facilitate the specification and control of the mobility behaviour without any impact on agent implementation. The paper advocates a policy-based model to specify mobility strategies separately from the agent code and presents a policy controlled mobile agents framework. This approach permits to adapt agent applications to the evolving conditions of the execution environment. The paper also presents a case study to validate our policy-based solution to agent mobility.