The Playful Invention and Exploration (PIE) Institute project was funded in 2005 by the National Science Foundation (NSF). For the past three years, Inverness Research has served as the external evaluator for the PIE project. The authors' evaluation efforts have included extensive observation and documentation of PIE project activities; ongoing in-depth interviews and discussions with project leaders and participants; a survey of PIE participants; and a review of the project's website. Overall, the authors have found the PIE work to be innovative, highly engaging and very meaningful to participants. It has also resulted in significant broader contributions to the field of informal science education institutions. In this report, the authors summarize the findings from their study of PIE, beginning with a brief description of the project and its activities, an analysis of the PIE philosophy and approach to teaching and learning, and an illumination of the key elements or defining characteristics of the PIE approach. They follow with a discussion of the contributions the PIE project has made to its participants: personally and professionally, to their programs, and to their institutions. They also discuss the important contributions the PIE project has made to the field of informal science education institutions. The third major section is a discussion of some of the challenges the project has faced, and opportunities going forward. (Contains 11 footnotes.) [For the executive summary, see ED508729.]