To be sustainable, ecotourism requires the protection of natural environments and processes both from development and operation of the tourism infrastructure, and from the activities of ecotourists within protected areas. This book chapter reviews the international literature on the study of visitor or recreation-related resource impacts with special reference to ecotourism. Four case examples are presented to characterize the geographic scope, focus, and principal findings of this recreation ecology literature and its relevance to ecotourism management. Case examples include the Cairngorms National Nature Reserve, Scotland; the Great Barrier Reef, Australia; the Central American tropics; and wildlife viewing in Kenya?s protected areas. Implications for the management of international protected areas and ecotourism resources are discussed.