This thesis is about what impairments qualifiy to be characterised as disability in Swedish social security legislation, and which principles are used to decide who is entitled to social security benefits as disabled. Through empirical case analyses the author shows that very different conclusions can demark a disability category in welfare policy. The analyses show that the definitions of this category are not always clear or sharp. Formal administrative principles and different underlying conceptions define a disability category in particular ways that bring different outcomes and demarcation lines of the category. The definition of a disability category is the result of contextual social processes and interpretations; disability as a social political and administrative category is in turn a result of specific social constructs and based on defined normative premises and cultural interpretations.