This study evaluates the cost-effectiveness of episodic and seasonal transportation control measures for ozone in two areas of Illinois. During the 1995-2000 period, twelve ozone alert days were issued in the Greater Chicago Metropolitan Area (GCMA), and five ozone alert days were issued in the Saint Louis Metropolitan Area in 2000, in anticipation of exceeding the 1-hour standard of 0.12 ppm. In July 1997, the U.S. Environmental Protection Agency (USEPA) revised the National Ambient Air Quality Standard (NAAQS) for ozone from 0.12 ppm to 0.08 ppm, and changed the standard from a 1-hour averaging time to an 8-hour averaging time. The 8-hour ozone standard has not been implemented yet by USEPA. Episodic and seasonal controls target emission reductions when those reductions are most valuable, during an "ozone episode" and "ozone season." Thus, episodic and seasonal control measures are expected to be more cost-effective than most fixed emission control measures, which reduce emissions on a continual basis throughout the year. A nationwide survey was conducted to find the most favorable, politically and economically, existing episodic and seasonal control programs. Based on the survey results, several episodic and seasonal control programs were identified as potentially effective and selected for further analysis. The analysis presented evaluates selected episodic and seasonal control measures for their cost-effectiveness and identifies the most cost-effective episodic and seasonal control measures.