There are numerous interventions worldwide targeting young people's participation in STEM. While gender imbalance seems to be a focus of many of these interventions, it is not clear how many specifically target females, and from those that do, what their characteristics and strengths are. We address this gap in the existing literature by conducting a systematic review of STEM interventions that target secondary school girls. Through the review, we identify thirty-two (32) interventions that we analyse to categorise their relevant characteristics, goals and evaluation approaches. This analysis reveals a lack of clarity around defining and describing the interventions themselves, as well as a wide variety of degrees of rigour when measuring and reporting intervention success. Using an intervention's ability to influence decisions to undertake STEM tertiary studies as a determinant of success, we uncover the importance of repeated or sustained engagement activities. Further, we find that successful activities enable STEM identity formation by combining inclusive curriculum and pedagogies with exposure to female role models. Finally, we argue that longitudinal evaluations of interventions have great potential to enrich both research and practice in this area.