There remains a serious structural disincentive to marry for many poor Americans. The U.S. tax and transfer (welfare) systems frequently impose substantial financial penalties on low-income couples who choose to marry. In relative terms, these marriage penalties tend to be much greater than those experienced by non-poor couples, and in some cases amount to family income losses of 20 percent or more. These marriage-discouraging financial penalties markedly undermine efforts to strengthen marriage among low-income Americans. This research brief provides a general introduction to the issue of marriage penalties and describes the uniquely high marriage penalty imposed on many low-income couples. The brief also features a new proposal to solve the problem. (Author abstract, modified).