Using three approaches, we attempt to explain changes in the Environmental Protection Agency's operating budget from 1970 to 2010. First, we compare qualitative predictions from two general types of agency lifecycle theories to the actual path of the EPA's budget. Second, we test empirically several hypotheses regarding the effects of political, economic, and other variables on the agency's budget. Third, we conduct detailed historical case studies of selected years. The results offer hints of support for theories predicting incremental change but do not support the hypothesized effects of external factors. We also uncover some historical anomalies that warrant further research.