This report summarizes the presentations, discussions and outcomes of the ECOOP’99 Workshop on Object Interoperability, held in Lisbon on Monday, June 14, 1999. Divided into four main sessions, the workshop covered some of the most important issues related to object interoperability at different levels (such as protocols, semantics, or mediated architectures). Two are the main goals of this report. First, it tries to provide a snapshot of some of the current research being carried out within the object-oriented community in these areas. And second, it summarizes some of the open questions and issues related to object interoperability, in order to set the basis for further research activities.