Social services are among the public policy areas criticized for lacking a reliable knowledge base to support professional as well as political ambitions and actions. This article contributes to the literature on knowledge perspectives in social service policies by studying and analyzing mechanisms that sustain a plurality of perspectives in the policies. The empirical material consists of knowledge perspectives in social service policies at the national level for child and family care and substance abuse treatment in Sweden between 1992 and 2015. Mechanisms that sustain a plurality of perspectives are identified with the support of an institutional logics framework. The main findings are that a plurality of knowledge perspectives. such as professional, scientific, and organizational, seems to be a permanent rather than temporary configuration; and that this permanent plurality is sustained by a set of mechanisms, including assimilation, blending, segregation, and contradiction. Despite this pluralism, there are few comments or guidelines in policy regarding the relationship between different knowledge perspectives. The findings suggest that more attention should be paid to the relationship between different knowledge perspectives and its impact on social work practice. In this, research and practice together need to support a development towards a more transparent professional acting.