Since the late-1990s, over 25 U.S. shared-use vehicle programs—including carsharing and station cars—have been launched. Given their presumed social and environmental benefits, the majority of these programs received some governmental support—primarily in the form of startup grants and subsidized parking. As of July 2003, there were a total of 15 shared-use vehicle programs, including 11 carsharing organizations, two carsharing research pilots, and two station car programs. Over the last five years, U.S. carsharing membership has experienced exponential growth.Despite this expansion, the social and environmental impacts and long-term sustainability of these services remain unclear. As part of their U.S. shared-use vehicle survey (August 2002 to July 2003), the authors documented market growth/trends and limited, systematic evaluation of program impacts. While 80 percent of shared-use programs implement internal customer surveys (initial or follow-up), only a handful of independent studies have been conducted to date. Across organizations, participant use and program benefits are measured using a variety of study tools and metrics. Given current shared-use vehicle growth and the ongoing interest of policymakers and government agencies in this concept, the authors recommend a longitudinal monitoring approach to better understand market developments, social/environmental impacts, and targeted policy strategies. Furthermore, the authors conclude that coordinated, program-wide data collection (consistent survey instruments and performance measures) could enhance overall market awareness and the credibility of shared-use vehicle organizations in leveraging additional public support.