Return on investment (ROI) has become part of the policymaking toolkit, particularly pertinent for activities like school-based career guidance deemed optional by some policymakers. There are institutions supporting ideal ROI methods alongside an academic critique, but little research on how ROI has been applied in practice in a guidance setting. In this systematic review, we document 32 ROI studies across nine countries that address either school-based guidance or one of three congruent fields: widening participation in education, behaviour in schools and adult career guidance. We find the corpus highly heterogenous in methods and quality, leading to problems in comparability. We argue for a pragmatic approach to improving consistency and the importance of policymakers' capacity for critically reading ROI studies.