Aims: The growing influence of implementation science has resulted in educational researchers exploring what occurs within schools to support intervention effectiveness. This paper provides an overview of existing research so that practitioners can understand the extent to which measures of implementation are associated with the outcomes of school-based interventions. Method: This paper systematically identified studies which correlated or directly compared the implementation of school-based interventions with pupil outcomes. Effect-sizes are reported and the strength of evidence appraised using a weight-of-evidence framework. Findings: The 13 studies reviewed reported 32 quantified effect sizes which represented the strength and direction of the relationship between measures of implementation and intervention outcomes in schools. The review also identified gaps in current evidence which have implications for further research and practice. Limitations: This review did not explore factors which supported staff to implement interventions effectively. As such, this review focusses on the effects of implementation, rather than detailed practices. Conclusions: This review found that educational researchers rarely measured fidelity of programme implementation. When fidelity is measured, there are indications that proper execution and co-ordination of evidence-based interventions is positively related to pupil outcomes. However, the measurement of implementation fidelity can be undermined when data is transformed into arbitrary categories, such as 'good' and 'bad'. The practicalities of effectively transporting evidence-based interventions into school settings are discussed.