Engagement in behavioral parent training (BPT), including enrollment, attrition, attendance, within-session engagement, and homework completion, has long been a critical issue in the literature. Several estimates of various aspects of engagement have been suggested in the literature, but a systematic review of the available literature has never been accomplished. This review examines engagement data across 262 studies of BPT. Recruitment attrition, program attrition, attendance, and within-session engagement are examined across studies, with particular emphasis on the impact that SES, study purpose (efficacy vs. effectiveness), treatment format (individual vs. group), and age of child may have on those rates. Results of this review suggest that the significant amount of attrition occurs prior to enrollment in BPT, with at least 25 % of those identified as appropriate for BPT not enrolling in such programs. An additional 26 % begin, but drop out before completing treatment. Still the combined dropout rate of at least 51 % leaves at best half of identified parents completing treatment. While SES status had a small effect on attrition, other variables were not found to meaningfully impact engagement. Information on within-session engagement (homework and ratings of participation) was not often reported in studies. Key issues in this literature (e.g., varying definitions of engagement, limited attention to reporting key aspects of engagement) are discussed, and recommendations are made to further improve this important area of research and clinical practice.