This article proposes a metamodel for describing the architecture of a High Level Architecture (HLA) compliant federation. A salient feature of the Federation Architecture Metamodel (FAMM) is the behavioral description of federates based on live sequence charts. FAMM formalizes the standard HLA Object Model and Federate Interface Specification. FAMM supports processing through automated tools, and in particular through code generation. It is formulated in metaGME, the metamodel for the Generic Modeling Environment.