European researchers have created a software standard named the Open Modeling Interface (OpenMI; http://www.openmi.org) that facilitates the integration of numerical hydrologic models with each other. Nearly 10 million euros have been invested in the development and testing of OpenMI through projects cofunded by the European Commission. Seven hydrologists representing various modeling-related initiatives in the United States traveled to Wallingford, United Kingdom, to learn about OpenMI and to discuss with European hydrologists the experiences and challenges inherent with developing integrated hydrologic models. The workshop was sponsored by the U.S. National Science Foundation (NSF) and the European Union (EU).