This volume constitutes the refereed proceedings of the Second International Conference on Interoperating Geographic Information Systems, INTEROP '99, held in Zurich, Switzerland in March 1999. The volume presents 22 revised full papers reviewed and selected for inclusion in the book. Also included are three invited full papers. The book addresses various topics of database interoperability and spatial data processing in particular identification, infrastructure, implementation, vectors and graphics, semantics, heterogeneous databases and representation.