This thesis examines the roles adopted by clinical pharmacists in the United Kingdom (UK), the evidence for the effectiveness of the interventions arising from these roles, and the challenges to and opportunities for future developments in clinical pharmacy. The research was undertaken in four phases: problem definition; national survey of services currently provided; in-depth interviews on roles; and a review of literature on effectiveness. A literature search provided background information on the evolution of pharmacy and of clinical pharmacy, particularly in the UK. These developments in have been set in the context of changes in health care provision and in the roles of the other health care professions and occupational groups. Preliminary interviews, meetings and group work were carried out to facilitate clarification of the research questions and to assist in the choice of methods. Two nationwide postal questionnaire surveys were conducted. One inquired about the provision of clinical pharmacy services to the primary care sector and the other about service provision within secondary care facilities in the National Health Service (NHS). The response rates were 91% and 90% respectively. The results show some diversity in the provision of clinical pharmacy services and provide possible explanations for this variation. Subsequently, semi-structured interviews were conducted with pharmacists, pharmacy technicians, doctors, nurses and managers at eight sites selected to represent different characteristics of hospitals. These qualitative data were analyzed by constant comparison. The results provide a picture of the clinical roles that hospital pharmacists are, and should be, providing. In addition, they indicate the potential barriers to, and opportunities for, future role development. An assessment of the evaluative literature on clinical pharmacy services was undertaken. Most literature is descriptive and much of the evaluative literature has shortcomings. The results present the evidence for the effectiveness of clinical pharmacy services in improving patient care and financial outcomes in the UK NHS. Finally, quantitative information gathered in the questionnaire survey, qualitative information from the interviews and the literature evidence were combined to create models of the future role of the hospital clinical pharmacist in the UK.