A major challenge for universities across the globe is to address the lack of diversity within higher education. Widening participation (WP) programmes can vary between in-reach and outreach programmes. Some initiatives focus on either pre-entry stage (primary or secondary schools) or post-entry stage (helping students adapt to university life). This systematic literature review provides an analysis of the effectiveness of widening participation outreach programmes, a topic that is relatively unexplored, particularly at school level education. By examining empirical studies of widening participation interventions, the paper allows for the investigation of the educational outcomes measured in these programmes. A total of 19 studies were included, which analysed quantitative or mixed-method data. The findings indicate a growth in the evidence base for widening participation programmes for second-level students over the last decade. Interventions identified in the literature were categorised by their approach, which included guidance-based interventions, multidimensional interventions or pedagogical interventions. The findings confirm some positive effects for widening participation outreach programmes on a range of outcomes including students' college readiness, educational aspirations and college enrolment. The review argues for consistency in the way in which educational outcomes are measured, as well as the need for cross-collaboration and sharing of data between schools and universities, to enable more effective measurement of widening participation programmes in future research.