The social work literature of the last two decades has drawn attention to the increasing importance of the assessment role of social workers. However, there has been limited historical analysis of the development of the assessment function in social work. This study examines the way in which the assessment role of social workers in England and Wales developed between 1950 and 1993. Historical narrative has been used to develop the themes of this study, addressing the significance of interrelated events and the range of political, economic and social influences on the changes which took place. It uses archival sources as primary material, including the archive of Lancashire County Council which is employed as a case study. This study traces the stages of development of social policy for the assessment function of social workers in local authorities. It draws attention to the diverse approaches of separate occupational groups of social workers to making enquiries and investigations about the circumstances of people who experienced social problems in the 1950s. Principles of social assessment which emerged during the 1960s are explored, together with their wider application to all client groups during the 1970s and the development of assessment as a priority activity within the newly established Social Services Departments (SSDs). The importance of the development of criteria and models of assessment in the late 1970s and 1980s and their inclusion as part of the social work process in planning documents produced by SSDs in the 1980s is discussed. The significance of the children’s and adults’ legislation and accompanying documentation at the end of the 1980s and beginning of the 1990s in giving local authorities a lead role in the co-ordination of assessments is addressed. Although conventional interpretations of the history of social work have suggested that a high point in the development of social work was reached in the early 1970s, archival material points to the function of social assessment being at a particularly early stage of its development. As a consequence of the changes which took place, the thesis argues that assessment was a unifying influence on social work as a single occupational group and emerging profession.