This work examines America's growing underground economy--its size, nature, and economic effects. The study begins with a critical analysis of the various methods used for measuring the size of the underground economy in the United States. It then develops a definition of the underground economy which is consistent with our system of national accounts. A sector-by-sector approach is used to measure the actual size of this sub-economy and to estimate the labor force involved. Finally, the author explores the economic consequences for the general economy of significant underground economic activity--for example: tax evasion, economic efficiency, and cyclical aspects. This work will be of interest to scholars, students, and policy-makers in economics, economic policy, political science, and criminology.