Self-regulation is positively associated with better academic, and life, outcomes. Consequently, many school systems aim to develop self-regulation, or related constructs. Thus, many teachers are asked to assess and report upon students' self-regulation (or related constructs). How secondary teachers in Ontario, Canada accomplish this task was investigated using mixed methods research. Phase 1 involved semi-structured interviews with 26 secondary teachers. The second phase of the study involved the analysis of report card data to examine large scale trends in self-regulation grades. The third phase employed an online survey whose development was informed by the interviews of the first phase. The overall findings are that Ontario secondary teachers vary in their definitions of self-regulation, but the strongest influences on teachers' self-regulation assessments are negative student behaviours. Report card data suggest that teachers struggle to assess self-regulation independently from other constructs such as initiative or collaboration. Implications for practice are discussed.