Abstract: Transferable development rights (TDRs) are a market-based approach to land conservation. They allow the development rights from one property to be transferred to another, with the first “sending” property placed under a development restriction or conservation easement and the “receiving” property permitted more dense development than would otherwise be allowed by baseline zoning regulations. This paper summarizes the economics literature on TDRs and describes a long-running program in a county in Maryland, one of the few programs with an active TDR market. It updates previously published results from the program and describes some problems that have arisen in recent years as the program has matured. The paper offers some observations as to why these problems have occurred and suggestions for other communities considering TDR programs.