In exploring the potential for mentoring to support novice teachers' use of effective teaching practices, we conducted a grounded theory analysis about change potential. Themes emerging from cross-case analysis of survey, interview, and observation data from six beginning primary teachers in the US and their mentors revealed factors, other than type of mentoring programme, that related to beginning teachers' success in improving classroom practices. More effective beginning teachers' mentors had more experience as mentors and were more effective teachers than other mentors. More effective beginning teachers communicated more with mentors, more accurately self-reported use of effective teaching practices, and were more open to mentoring.