A major challenge for managers of parks and other conservation lands where recreation is allowed is to ensure that visitors do not impair the natural values for which those lands were established. Recreation ecology is the academic discipline that provides a scientific foundation for managing the ecological impacts of outdoor recreation use. This article traces the development of recreation ecology from its disparate beginnings in the early 20th century, through a period of rapid growth starting in the 1970s, until its early maturity by the end of the 20th century. It introduces the reader to early recreation ecologists, such as E.P. Meinecke, Neil Bayfield, and Michael Liddle, and describes the important early investments in this work by US Forest Service Research. It reviews some of the most important early applications of recreation ecology: inventory and monitoring techniques, the Leave No Trace education program, and knowledge about how impact varies with factors that are subject to management control (e.g., amount of use, type of use etc.).