How do we measure the impact of an intervention aimed at an entire sector? The Ontario Centre of Excellence for Child and Youth Mental Health (the Centre) adopted a developmental evaluation approach (Patton, 2010) to address the complexities of its stakeholders’ operating systems. The Centre strives to improve child and youth mental health (CYMH) sector effectiveness through building agency capacity in implementing and sustaining evidence-informed practices. The Centre’s work is unique as it seeks to influence the practices of the entire field, through what is referred to as a ‘system-level intervention’ or ‘systems change’ (Hargreaves, 2010). Both of these concepts are subsets of the broader systems theory (Sterman, 2000). The Centre’s adaptation of a developmental evaluation approach resulted in a performance measurement framework with the ability to measure the Centre’s integrated impact on the entire CYMH sector, while also capturing programmatic outcomes and the nuanced nature of the ever-changing sector. #SystemsinEvaluation #2012Conference