This investigation seeks to understand what constitutes successful professional learning for informal educators, and thus contributes to the research base needed for developing a professional infrastructure for the informal education field. This study draws on summative evaluation research for the national field trial of a professional learning (PL) program intentionally designed for informal educators. Seventeen informal educators from ten informal science education institutions share their experiences implementing the program with their colleagues, and reflect upon the extent to which they believe the program was successful. Interview data reveal there are four changes in educators' practice that can be used to indicate success of the PL program, and several key supports that help ensure success. Findings from this investigation offer insights for developing the practice and profession of informal educators, specifically, and for advancing the field, in general.