MASIF is a standard for mobile agent systems which has been adopted as an OMG technology. It is an early attempt to standardize an area of industry that, even though popular in the recent past, still has not caught on. In its short history MASIF has raised interest in industry and academia. There are already a number of projects pursuing MASIF reference implementation. MASIF addresses the interfaces between agent systems, not between agent applications and the agent system. Even though the former seem to be more relevant for application developers, it is the latter that impact interoperability between different agent systems. This paper describes two sets of interfaces that constitute MASIF: MAFAgentSystem and MAFFinder (the acronym MAF is used for historical reasons). MASIF extensively addresses security. The paper provides a brief description of MASIF and its interfaces, data types and data structures.