BACKGROUND: Residents are at the forefront of student education in the hospital, yet valid tools to assess their performance as teachers are lacking. AIMS: To develop a valid evaluation tool for assessing resident performance as educators for clerkship students. METHOD: A mixed-methods design was used. Focus groups of residents and medical students explored desired behaviors in resident educators. Using grounded theory, a list of behaviors was generated inductively through iterative review and categorized into themes. After thematic saturation, behaviors were rated on a Likert scale by stakeholders based on "importance" and "accuracy of measurement." Items which were both important and accurate were used in the final tool. RESULTS: Eighty-five desirable behaviors for resident educators were identified and consolidated into a 14-item tool. Twenty met both "importance" and "accuracy" criteria and fell under themes of respect, safe environment, balancing supervision with autonomy, relevant teaching and feedback. Nineteen "important" behaviors deemed not accurately measurable fell under themes of professionalism, communication, management skills and leadership. CONCLUSIONS: Evaluation of residents as teachers and development of resident-as-teacher curricula should emphasize aforementioned areas. Professionalism and organizational skills may not be measurable reliably by learners. Complementary tools to assess these aspects of resident performance are necessary.