Recent evidence indicates that the proportion of young people developing significant psychosocial problems is increasing (Rutter, 1995). This thesis focuses upon a small percentage of the growing number of adolescents who experience significant psychopathology; adolescents admitted to a regional inpatient psychiatric unit. Chapter 1 provides the theoretical background to the thesis; reviewing the literature on both the methodology and outcome of studies conducted with adolescents in inpatient adolescent psychiatric services. It suggests that future investigations take account of the practical impossibility of conducting high quality, standard double-blind control trials in such settings and outlines four empirical alternatives to such designs; evaluation of components of the therapy employed in residential settings; the use of problem-oriented case records; prediction of outcome across time; investigating the reasons for the effectiveness of established predictors of outcome. The next four chapters describe a series of studies utilising the last three of these suggested approaches in a cohort of adolescents admitted to a regional psychiatric unit. Chapter 2 is a cross-sectional study describing the sample upon which the remainder of the thesis is based. It focuses principally upon describing the psychopathology of the adolescents from three perspectives; that of the adolescent, their parents and the therapeutic team. It examines the interrelationships between mood, behaviour and family functioning. Chapter 3 describes the results of an investigation into the relationships between selfesteem, autobiographical memory, attributional style, social problem-solving and the experience of depression and hopelessness. Chapter 4 examines the clinical effectiveness of the treatment at the unit as assessed by problem severity as rated by adolescents and their parents and standardised measures of mood and self-esteem. Chapter 5 examines to what extent outcome can be predicted on the basis of initial problem severity, mood and cognitive style (autobiographical memory, attributional stye, problem-solving and selfesteem). Each chapter is written as a separate paper with abstract, literature review, method, results and discussion; however, reference to the overall findings is made throughout for ease of reading. The final chapter draws together themes from the whole thesis and discusses the psychological processes which mediate change during the adolescent treatment programme described here. It concludes that the cognitive and meta-cognitive processes which mediate change are best understood as related to internal working models derived from attachment experiences. A model for understanding individual variation in these processes is outlined and strategies for testing the proposed model are advanced.