A proposed set of aviation system performance measures that have been defined to support multimodal transportation planning in California is presented. As part of the 1998 update of the California Transportation Plan (CTP), the California Department of Transportation has defined a framework of system performance measures that could be applied on a multimodal basis to assess the performance of the transportation system, support informed decision making, and establish a coordinated and consistent process for performance measurement throughout the state. The research that is described examined approaches to measuring the performance of the aviation system that have been adopted at a national level and in other states, as well as general principles for measurement of transportation system performance that emerged from a conference held in 1997 as part of the CTP update process. The range of considerations that arise in measuring the performance of the aviation system is discussed, and potential performance measures that have been proposed to address the requirements of the CTP framework are identified. Finally, issues that will need to be addressed in implementing any comprehensive set of performance measures for the aviation component of the transportation system are discussed.