As control strategies for both mobile and stationary sources have been implemented and have become effective, they have caused a significant reduction in California's air pollution problem. New strategies are focusing on heretofore uncontrolled emissions sources. The California Air Resources Board has supported a study to assess the current and future significance of currently uncontrolled mobile and semimobile sources of emissions powered by internal combustion engines. The two primary goals of the study were to develop an emissions inventory for those emissions sources in California not presently controlled by existing legislation and regulations and to make an accurate determination of the contribution of these sources to California's total air pollution problem. This paper focuses upon the results of the ARB-sponsored study as it relates to farm and off-road heavy-duty equipment. It includes the estimation of vehicle and equipment populations, horsepower and engine types, growth trends, and various other relevant characteristics associated with the uncontrolled emissions sources for the State of California.