In this article we report on our examination of the challenges faced by four successful and long-standing national STEM reform communities. Drawing primarily on interview data from a large-scale, multi-year study informed by literature on "communities of practice" (CoPs) (Wenger et al. 2002), we describe five categories of challenges faced by the communities and the solutions employed to overcome them in order to sustain themselves and meet their goals. We chose to focus on these large and dispersed CoPs because, although on-campus CoPs have received some scholarly attention, no research has been conducted on national or regional CoPs. Based on our findings, we conclude that the solutions used to address these challenges reflected a meta-theme of flexibility in matters of design, leadership, and decision-making that contributed to the communities' success and longevity.