The United States is increasingly participating in coalition military operations.Coalition support may be required for successful military operations and in most such operations the United States desires to share the burden. U.S. allies recognize the increased security that coalition operations can bring. Because interoperability is a key element in coalitions, RAND undertook research to help the Air Force identify potential interoperability problems that may arise in coalition air operations and to suggest nonmateriel and technology-based solutions. The research focus is on command, control,communications, intelligence, surveillance, and reconnaissance (C3ISR) systems in out-of-NATO-area operations. The authors' review of recent coalition air operations found that interoperability problems arose because of differences in doctrine, incompatible communications, different planning and execution systems, and different weapon system capabilities. For example, allies may lack sufficient all-weather, day and nightprecision-guided weapons. The authors suggest the following to increase interoperability in coalition operations: (1) common or harmonized doctrine for combined joint task force operations, from planning through assessment, (2) compatible or adaptable concepts of operation for airborne surveillance and control, (3) common information-sharing standardsand compatible tactical communication systems, and (4) expert, experienced personnel who understand the capabilities of coalition partners. From a technology perspective and cost considerations, C3ISR initiatives appear to offer the best opportunities for interoperabilityenhancements.