In 2005-2006, the National Park Service Office of Interpretive Planning at Harpers Ferry Center, in collaboration with the author, conducted an evaluation project to (a) assess the appropriateness and quality of specific elements of National Park Service (NPS) interpretive plans, (b) determine where improvements in planning might be made, and (c) support the continued improvement of interpretive planning services within the agency. This article describes the process of evaluating a set of long range interpretive plans developed by the National Park Service. Based on the results of the evaluation, the article concludes with a set of reined interpretive planning principles proposed by the author. (Contains 2 figures, 1 table and 4 notes.)