This guidebook is intended to inform Federal agencies on their opportunities to purchase renewable energy. Definitions of renewable energy and green power are discussed and potential benefits and costs of Federal renewable energy purchases are identified. In addition, procurement issues and approaches are identified that are appropriate for both regulated and restructured markets. Drawing upon lessons learned from agencies that have recently completed or are currently pursuing renewable energy purchases, key elements of a successful renewable energy procurement are outlined. Agencies that are interested only in participating in procurements run by the General Services Administration (GSA) or the Defense Energy Support Center (DESC) should turn directly to Section 5.3 of this document.