This article reports the recent experiences of single-sex teaching in 31 co-educational English comprehensive schools, often implemented because of the perceived need to raise boys' achievement. Since it was often undertaken on a short-term basis, its effectiveness is difficult to evaluate, although some positive aspects were noted in some schools in terms of raised achievement levels and increased confidence and participation in class. In other schools, however, single-sex teaching appeared to have little impact on achievement levels and led to increased problems of behaviour management in boys' classes, with male bonding between male teachers and male students reinforcing the qualities associated with hegemonic forms of masculinity. The authors conclude, therefore, by suggesting that single-sex classes can provide a positive and successful experience for girls and boys where there is strong commitment from staff, a willingness to evaluate and to diffuse good practice, but crucially where gender reform strategies are in place to challenge any practices and behaviours that reinforce stereotypical gendered roles.