Implementation refers to the process by which an intervention is put into practice. Research studies across multiple disciplines, including education, have consistently demonstrated that interventions are rarely implemented as designed and, crucially, that variability in implementation is related to variability in the achievement of expected outcomes. Put simply, "implementation matters" (Durlak & DuPre, 2008). This paper reviews several key issues in the study of implementation and calls for an increasing emphasis on this often neglected aspect of evaluation research in UK journals. These issues include programme-specific reasons for studying implementation as an intervention passes through the various stages of development, advancing knowledge and understanding about the processes of implementation (including the balance required between fidelity and adaptation, and the range of factors that may facilitate or impede implementation), and improving measurement and assessment of implementation. Through discussion of these issues the case is made for more research that focuses specifically on the examination of implementation in school settings.