This thesis addresses the issue of standardization in social work. Standardization is a necessary and fascinating, though underestimated, phenomenon, which governs, regulates and calibrates social life. In Swedish social services, we have witnessed an increased use of top-down imposed manual-based tools for investigative and assessment work. In human service organisations, such as social services, this raises questions about social workers’ control in terms of how work should be executed as well as the implications of standardization for professional discretion. The aim of this thesis is to investigate and understand standardization as a phenomenon, focusing on its consequences for social work as a profession and a field of practice.The thesis consists of four papers investigating the implementation and execution of manual-based assessment tools (BBIC, FREDA and SARA) in social services. The empirical material is based on interviews with a total of 68 social workers, managers, politicians and officials as well as documents and observations of risk assessment events. Each paper results from the analysis of an exclusive data set, with the concepts used in the analysis coming from theories of professions and organisations.The results show that increased standardization is a way for social workers to seek legitimacy as well as to claim jurisdiction and increase professionalism. The execution of the tools in social work is conditioned by significant key implementation factors in organisational contexts as well as the compatibility between the construction of tools and users’ needs and expectations. Along with previous research and theory, the results from the four papers are used to develop a tentative taxonomy of different discretionary positions that social workers can take with regards to standards in their practice. Those positions illustrate that it is not straightforward how the increased standardization of investigative and assessment work will impact professional discretion. It is argued that a balance between standardization and professional discretion is possible.