Military Keynesianism has positive supply-side and demand-side effects. However, it also has negative aspects, which make it difficult to sustain over an extended period. Might penal Keynesianism serve as a replacement? The prison system today draws from the pool of young adults with low levels of education and poor employment prospects, individuals who earlier might have found "employment' in the military. There is a movement to increase employment of prisoners—in both prison and private-sector projects. And the number of prisons and the amount of total justice expenditures are seeing significant growth. This paper describes military Keynesianism, examines the evolution of the justice system, and contrasts penal Keynesianism with military Keynesianism in terms of labor supply, technology and infrastructure, and demand stimulus. I conclude that penal Keynesianism is a poor substitute for military Keynesianism, and I offer an alternative to both, in the form of a nation-wide program of public service employment that would target low-skilled, poorly educated workers.