Effective health information systems require timely access to all health data from all sources, including sites of direct care. In most parts of the world today, these data most likely come from many different and unconnected systems-but must be organized into a composite whole. We use the word interoperability to capture what is required to accomplish this goal. We discuss five priority areas for achieving interoperability in health care applications (patient identifier, semantic interoperability, data interchange standards, core data sets, and data quality), and we contrast differences in developing and developed countries. Important next steps for health policy makers are to define a vision, develop a strategy, identify leadership, assign responsibilities, and harness resources.