In 2012, the Adverse childhood experiences (ACEs) Public-Private Initiative (APPI), a Washington state consortium of public agencies, private foundations, and local networks, was formed to study interventions to prevent and mitigate ACEs and facilitate statewide learning and dialogue on these topics. As part of the initiative’s cross-site evaluation, the evaluation team sought to assess the extent to which five community sites developed sufficient capacity to achieve their goals and examine the relationship of the sites’ capacity to selected site efforts and their impact on ACEs-related outcomes. The team conducted an extensive review of the research literature focused on community capacity development. Based on this review and in collaboration with the APPI sites, the team created an instrument to measure the APPI sites’ collective community capacity to address ACEs and increase resilience in their communities: the ACEs and Resilience Collective Community Capacity (ARC3) instrument.