Inverness Research Associates served as external evaluators for the Center for Informal Learning and Schools (CILS) from its inception in 2002 as a National Science Foundation (NSF)-funded Center for Learning and Teaching. One of the programs that CILS developed was the Informal Learning Certificate (ILC) for informal science educators (mostly museum-based) who work with K-12 schools. Through this three-year commitment, which involved face-to-face workshops, conferences, and online communication, the ILC program aimed to provide participants with intellectual and professional grounding, community, and a range of tools to improve their work with teachers, students, and school systems. Three cohorts totaling 84 participants completed the ILC program. Throughout the five years that CILS operated, Inverness Research conducted formative and summative evaluation activities both center-wide and specifically focused on the ILC. This report presents a summary of Inverness' view of the ILC, its impacts, challenges, and future opportunities. The report comprises the following four sections: (1) Introduction, contains (a) National Context--a dearth of professional development opportunities for informal science educators; and (b) Background about CILS and the ILC. (2) Program Impact: The Value and Benefits to ILC Participants, their Institutions, and the Field; (3) Challenges, Lessons Learned, and Opportunities Going Forward; and (4) Appendix: Participant Vignettes--Rita Bell and Preeti Gupta. The intended audience for this report includes science educators, museum educators, professional development providers, teachers, reform leaders, funders, and the general public.