Systematic reviews of offenders’ as well as non-offenders’ engagement research revealed inadequate and inconsistent definitions and assessments of engagement and an absence of theory. Furthermore there is no research on facilitators’ engagement in offending behaviour programmes. A constructivist grounded theory methodology was employed to develop a theory of engagement in group offending behaviour programs that accounts for facilitators’ engagement as well as that of offenders’. Interviews and observations of sessions were used to collect data from 23 program facilitators and 28 offenders (group members). Group members’ engagement was a process of ‘moving on’, represented by a number of conceptual categories including early ambivalence, negotiating the group, and acknowledging and accepting. Facilitators’ engagement was a process of building engagement, by personalizing treatment frameworks using ‘the hook’, a cornerstone of treatment similar to the therapeutic or working alliance. It also involved disarming group members and dealing with initial resistance, and establishing roles and positions in the treatment framework. There were a number of barriers to both group members’ and facilitators’ engagement identified that were rooted in programme and referral factors. The TEGOBP provides four distinct developments in engagement research as well as a number of important implications for research and practice that are discussed.