Behavioural, emotional and social difficulties (BESD) can develop early in a child's life, and the earlier the onset the poorer the prognosis. Classroom-basedin terventions have been effective in reducing and preventing these problems, but many do not possess a robust evidence-base. One series of programmes that has a tradition of scientific evaluation is the Incredible Years (IY) series; the IY Teacher Classroom Management (TCM) programme is the focus of this research. This thesis contributes to current research in the following ways: Firstly, by developing and refining a new classroom observation measure (Chapter 4), the Teacher-Pupil Observation Tool (T-POT); a relatively flexible measure of teacher, classroom and individual pupil behaviours and interactions. Secondly, study two (Chapter 5) utilised the T-POT - in combination with questionnaire measures - in order to evaluate the IY TCM programme. The contribution of this study was that the IY TCM programme had never been previously evaluated independent of other IY programmes. The TCM programme successfully increased positive teacher behaviour and decreased negative teacher behaviour to the classroom, and to children with behaviour problems in particular. Pupil compliance significantly increased as a result of TCM training, while non-compliance, deviance (especially in children rated as problematic by the teacher), negative behaviours aimed at the teacher, and off-task behaviour showed significant reductions in intervention classrooms, post-classroom management training. Thirdly, the final study (Chapter 6) contributed to current research by investigating mechanisms of intervention-related change. This study examined barriers to positive outcome and investigated factors that facilitated implementation of TCM skills and principles. Teacher experience, job-share status, and teacher stress level, predicted multiple variables. Broad implications of the findings are discussed in the final chapter (Chapter 7) and suggestions relevant to future research are made.