GXL (Graph eXchange Language) is an XML-based standard exchange format for sharing data between tools. Formally, GXL represents typed, attributed, directed, ordered graphs which are extended to represent hypergraphs and hierarchical graphs. This flexible data model can be used for object-relational data and a wide variety of graphs. An advantage of GXL is that it can be used to exchange instance graphs together with their corresponding schema information in a uniform format, i.e. using a common document type specification. This paper describes GXL and shows how GXL is used to provide interoperability of graph-based tools. GXL has been ratified by reengineering and graph transformation research communities and is being considered for adoption by other communities.