A teacher's conceptions and beliefs about the way mathematics is learnt determine his/her approach to the teaching of mathematics. Many teachers in Pakistan conceive of mathematics teaching as the memorization of mathematical procedures and formulae by students, to be regurgitated during tests and examinations. Such a situation may result in students becoming passive learners and the teacher taking over their mathematical thinking. A constructivist philosophy of knowledge and learning, which is rapidly gaining credence in educational circles, calls for learning to be a self-constructed process for the student. This study, therefore, explored the impact of mentoring teachers in approaches compatible with a constructivist philosophy. Data were collected mainly through classroom observations and interviews with teachers and students. Findings revealed that teachers did in fact come to class with certain beliefs and conceptions about the way children learn mathematics, that the origins of these beliefs and conceptions lay in their prior experiences as students, and that the way they taught mathematics was a result of these beliefs. The findings also revealed that as a result of mentoring and critical reflection on practice, teachers could change these conceptions and their approaches to the teaching of mathematics.