Drawing on interviews with fourteen teachers from three different secondary schools, this thesis examines the importance of critical episodes in the formation of teachers' beliefs and practices. Teachers nominated a variety of experiences as being critical, from episodes that had occurred during their own schooldays, to their most recent classroom occurrences. These episodes were found to be crucial in determining their self image of the type of teacher they were. Some episodes involved formal professional development activities or post graduate study, and thus incorporated a theoretical basis for the beliefs and practices, but most were firmly rooted in the everyday lives of the teachers, and their experiences within and beyond the classroom.