The need to understand the complexity and global and systematic nature of social issues implies that policies are rarely straightforward. Their objectives generally fall into various rationales. This reality questions the way in which a social policy is evaluated: what effectiveness means, and how can it be measured for a policy with multiple objectives for beneficiaries with multiple expectations? This chapter proposes an appropriate theoretical framework for understanding and explaining the different meaningful reference points which underpin social policy: the framework of the economies of worth of Luc Boltanski and Laurent Thévenot. In view of this theory, such a policy and its evaluation are part of a specific plan of action: public justification; that is, they must respond to the principles of justice that make them credible and justifiable. These principles of justice refer to different concepts of the common good and give meaning to different types of actions that are highlighted and expressed when evaluating public policy. This chapter is divided into three sections. First, the rationale of transformation of the societal context in which public and social policies unfold and the implications for the design and practice of evaluation will be briefly characterized. Next, the theoretical framework for understanding multiple rationalities and principles of justice will be outlined. Finally, this framework will be applied in a heuristic perspective to the literacy policy of the Wallonia-Brussels Federation (the French-speaking part of Belgium). This application of convention theory demonstrates its relevance to the evaluation of public policy.