In social work, the methods for achieving policy goals are often subject to some degree of local and case-by-case autonomy. This autonomy enables the design of interventions to be negotiated between different actors, which are underpinned by diverse logics, interests, and knowledge-bases. This dissertation explores the reasoning behind social work intervention design. The study focused on how involved actors reason when they deliberate on which design to choose inindividual cases, including how different forms of knowledge and local organizational conditions affect the choice of design. The aim was to develop comprehensive knowledge about social work intervention design that is not limited to the influence of a single actor, knowledge form, decision-making model, or specific organizational condition. The study found that professional experiential knowledge and the local availability and range of intervention forms are the most significant factors shaping the design and customization of intervention. Furthermore, the study found that the local range of intervention forms is not based on professional analysis, but rather developed randomly and subject to managerial control. Client experiences can be significant if social workers assess their desires as credible and sustainable, or if clients are highly motivated or have had success in the past with the intended form of intervention. Several other factors of significance exist, but to a lesser degree or in a smaller proportion of the studied municipalities, such as financial constraints, the community interest of laypersons, and research findings. Regarding the use of research-based knowledge, there seems to be a decoupling between management and street-level. Social work managers described research-based knowledge,evidence-based practice, professional experiential knowledge, procurement, and legislation as the most significant factors of treatment deliberation.