We report on the origins, development and refinement of an online inventory to help cooperating teachers focus on selected dimensions of their practice. The Mentoring Profile Inventory (MPI) helps quantify important features of both the motivating and challenging aspects of mentoring student teachers and provides results to respondents in a graphic, easy-to-understand and immediate feedback report (14 sub-scales and 3 summary charts). Psychometric properties of the MPI are shown to be robust. Results can be used individually or collectively to facilitate cooperating teacher professional development by providing the opportunity for dialog around a set of common issues. (Contains 2 figures and 7 tables.)