Over the past seventy years Congress has enacted dozens of tax and transfer programs, giving little if any attention to the marriage subsidies and penalties that they inadvertently impose. Although the programs affect both rich and poor Americans, the penalties fall most heavily on low- or moderate-income households with children. In this article, Adam Carasso and Eugene Steuerle review important penalties and subsidies, explain how they work, and help fill a big research gap by beginning to provide comprehensive data on the size of the penalties and subsidies arising from all public programs considered together. [ www.futureofchildren.org]