While there is widespread agreement about the need to evaluate professional development, actual evaluation remains rare, despite the fact that K-12 school districts invest in professional development as the main mechanism to grow teacher practice and improve student learning. This article explores research literature about why current mechanisms for evaluating professional learning are under-utilised and offers an alternative mechanism derived from a recent study of professional developers' perspectives about and experiences of evaluating the professional development they lead. The features of the mechanism and the personal and systemic conditions that make its use more or less likely emerged from a ground theory study of professional developers in Saskatchewan, Canada. The resulting theoretical framework describes the process K-12 professional developers use to assess their work, including how they select what they will assess, their processes for data gathering and interpretation, and how they take action based on what they find. Additional findings identifying personal and systemic factors that encourage or discourage assessment may support teachers, schools, central offices and policy-makers to increase the frequency, focus, and quality of evaluations of teacher professional learning.