Since the 1970's, an increasing number of State and local governments have administered purchase of development rights programs (PDR's) that use easements to protect farmland and similar lands from conversion to other land uses, such as residential development. The economic principles, issues, and problems underlying these farmland protection programs are broadly examined, with an emphasis on basic design and implementation aspects of easement programs. Other issues, such as the role of private land trusts in farmland protection, are also discussed.