This paper presents findings from a literature review on relationships and sexuality education for primary (elementary) school programme effectiveness. Using a theory-generating approach informed by realist evaluation, the review takes a deep dive into the literature to identify contexts, mechanisms and outcomes likely to influence the effectiveness of programmes. This paper will be of interest to researchers, evaluators, RSE programme designers and practitioners as findings provide a catalogue of factors (variables) that can be considered for application in future programme evaluations and for inclusion in programme logic models. It identifies areas of opportunity to incorporate children's voices into research and points to the need for longitudinal studies to address the context- and time-sensitive nature of relationships and sexuality education.