This chapter discusses implementation science in relation to the process of evaluation of effects in social and educational interventions. Many social policy interventions delivered in education, public health practice or family and children services can be viewed as 'complex interventions'. Adequate development and piloting work needs to be incorporated, with specific attention being paid to the practical issues of implementation. Once an issue has been identified for resolution, the existing evidence base should be identified in order to select a programme or intervention with evidence of effectiveness for the target population. Complex interventions may include many different participant levels, such as individuals (e.g., pupils, teachers and parents), community units (e.g., children centres or schools), or whole populations. Recruitment and retention are likely to be higher if the intervention is valued by potential participants such as teachers, children, parents and delivery staff.