The concept of mobile agents promises new ways of designing applications that better use the resources and services of computer systems and networks. For example, moving a program (e.g. search engine) to a resource (e.g. database) can save a lot of bandwidth and can be an enabling factor for applications which otherwise would not be practical due to network latency. Leveraging on the strengths of Java, several Java-based mobile agent platforms became available recently. This report introduces features Java-based mobile agent platforms should provide in our view and investigates in detail the current versions of three existing platforms. The aim is to provide developers with a detailed comparison and to help in selecting an appropriate platform. Keywords: Mobile Agents, Mobile Code, Platforms, Java