This article attempts to contribute to the debate on how to define and theorize institutions. particularly regarding normativity and explanations for conformity. Firstly, it proposes some distinctions and concepts: it separates moral from epistemic values, leading to different types of legitimacy and social norms; then it distinguishes different meanings of the term 'normative' and introduces the concept of decision-theoretic norm. Secondly, the article defends a broad concept of institutions by arguing that some institutions are neither social norms nor decision-theoretic ones, a point that matters for institutional change and stability. Some conventions from which innovators break are highlighted as an example. (c) 2009 Elsevier B.V. All rights reserved.7217078