This study used interviews and other artifacts collected during student teaching as the basis for rating 13 elementary cooperating teachers on their supervisory effectiveness. Once highly effective cooperating teachers were differentiated from their less effective peers, researchers used ex post facto methods to identify background and intervention factors associated with their effectiveness levels. Three factors consistent in the highly effective group were (1) being midrange in number of teaching years, (2) having supervised more than five earlier field experience students, and (3) having closely collaborated with the university supervisor. However, the most powerful association for high effectiveness was the graduate-level preparation in supervision. Four of the five most effective teachers in this study had master's degrees in teacher leadership, and all had taken course work on systematic observation and feedback, as well as conferencing skills. This deep preparation was associated with an ability to articulate beliefs behind practices and use practices congruent with those beliefs. (Contains 2 tables.)