One way to improve the quality of family engagement in K-12 U.S. public schools is by ensuring beginning teachers are well prepared to engage with their students' families. To ensure this, teacher educators and policymakers first need to know what works in preparing preservice teachers for effective family engagement. This systematic review quantitatively and qualitatively describes the existing evidence base. We collated and analyzed the quality of peer-reviewed empirical studies that evaluated interventions designed to improve preservice teachers' capacity to work with parents/guardians. We found a total of 25 studies (8 qualitative, 6 quantitative, 11 mixed methods). Although together these studies provide correlational evidence of improvements in preservice teachers' "beliefs" and "self-efficacy," only one study was designed to provide rigorous causal evidence of improvement in preservice teachers' "skills" for engaging with families. These findings suggest that future research should employ more rigorous evaluative research designs and more practice-based outcome measures to strengthen the evidence base on preservice family engagement interventions.