Standards for new cars and trucks have dramatically reduced the emissions of criteria pollutants (HC, CO and NOx) from new vehicles. However, the standards have not resulted in comparable reductions in in-use emissions from vehicles as they age and are operated under ”real world” conditions. Inspection and Maintenance (I/M) programs (called Smog Check in California) are the keystone of EPA's strategy to reduce in-use vehicle emissions. We have performed evaluations of EPA's recommended I/M program in Phoenix, as well as California's Enhanced Smog Check program, using both program test results and independent measurements of on-road emissions. The evaluations have revealed some important shortcomings of the programs. This presentation will provide an introduction to vehicle emissions and their regulation, findings from evaluations of the two I/M programs, and suggestions to improve their effectiveness.