This thesis is concerned with the development and evaluation of the collaborative care model for the treatment of depression in primary care in the National Health Service (NHS). It begins with an outline of the problems experienced by people with depression and the challenge that their effective care presents to the healthcare system. It then considers the response of evidence-based medicine to this challenge and briefly reviews the commonly used tools of evidence-based medicine (such as systematic reviews and clinical guidelines). The origins of the collaborative care model in the treatment of chronic physical health problems are then reviewed to provide a context for a subsequent review of the current evidence for collaborative care for depression. Following this review the main elements of the effective treatment of depression are examined and include systematic reviews of the major psychological and pharmacological treatments. The competences required to deliver low-intensity brief psychological interventions are also developed and described. The design, implementation and evaluation of an exploratory trial of collaborative care of depression are then described and set in an overall framework for the evaluation of complex interventions as outlined by the Medical Research Council. The outcomes of the trial and a parallel process evaluation are then presented and the limitations of the trial considered. The implications of the trial for the future development and evaluation of collaborative care in the NHS are discussed.