Widening participation (WP) in higher education (HE) is an increasingly important policy issue, with interventions to increase participation from minority ethnic, low-income and other under-represented groups undertaken in HE sectors in many countries. In the UK there is a large amount of WP activity but a lack of robust evidence of its effectiveness. This article presents a systematic review in the topic area of WP in HE. We included studies of systematic review, randomised controlled trial (RCT) and quasi-experimental (QE) designs, and assessed evidence of the effectiveness of university access strategies and approaches in relation to the participation of disadvantaged students at university. We searched for, quality appraised and synthesised the international evidence, that is, evidence published in any country, in the English language. The findings from 4 systematic reviews and 12 experimental studies (4 RCTs, 4 RDDs and 4 QEDs) are presented as narrative syntheses in a series of thematic sub-topics. We found some evidence of effectiveness for a number of university access interventions. 'Black box' WP programmes (those with multiple elements in a single programme) and financial incentives were found to be effective. However, much of the evidence had design limitations and the majority were conducted in the US. The article concludes with research recommendations in relation to UK interventions, including suggested designs for future quasi-experimental evaluation.