Classroom observation of four student teachers during the first, seventh, and fourteenth weeks of their teaching semester focused on their management of transition periods between lesson segments. Videotapes and stimulated recall analysis of the differences revealed that: (1) the most effective student teachers averaged fewer transitions per lesson; (2) the less effective teachers' transitions were more than three times the duration of their more effective counterparts; (3) the more effective teachers devoted less overall class time to switching activities; and (4) effective student teachers had no behavioral problems requiring desists during their transitions while the less effective teachers averaged 3.3 desists per transition. Suggestion are made for making transition periods more efficient. (JD)