Evaluating the impact of complex whole-school interventions (CWSIs) is challenging. However, what evidence there is suggests that school leadership and other elements of whole-school contexts are important for pupils' attainment (Leithwood et al., 2006), suggesting that interventions aimed at changing these have significant potential to improve pupil outcomes. Furthermore, strong leadership is likely important for the effective implementation of many interventions funded by the Education Endowment Foundation (EEF) since even class-level or targeted programmes are more likely to work best within supportive and effective settings. Developing design and practice for evaluations of this type of intervention, focusing on the issues of complexity and managing change across a whole school, increases the scope of projects of which the EEF may confidently fund evaluations. In this document, the authors provide key messages for EEF evaluators on how to get the most out of evaluations of CWSIs, including considerations for both design and implementation. As far as possible, their suggestions aim to be practical steps that evaluators can implement immediately. [This report was produced with the Behavioural Insights Team.]