This paper examines transferable development rights (TDRs) policies as a way to preserve farmland and change the density of development. Characteristics of TDR markets are described, including why they might promote efficiency, and the difficulties that arise in implementing them. Evidence from an established TDR program in Calvert County, Maryland, is used to assess the potential for TDRs to influence subdivision density, and to achieve local land preservation goals. The Calvert program has succeeded in creating an active and stable TDR market, and has therefore preserved a large amount of farmland in the region. But we find that the demand for additional density permitted with TDRs occurs mostly in rural areas and not in the higher density town centers and residential areas.