The legitimacy and accountability of polycentric regulatory regimes, particularly at the transnational level, has been severely criticized, and the search is on to find ways in which they can be enhanced. This paper argues that before developing even more proposals, we need to pay far greater attention to the dynamics of accountability and legitimacy relationships, and to how those in regulatory regimes respond to them. The article thus first seeks to develop a closer analysis of three key elements of legitimacy and accountability relationships which it suggests are central to these dynamics: The role of the institutional environment in the construction of legitimacy, the dialectical nature of accountability relationships, and the communicative structures through which accountability occurs and legitimacy is constructed. Second, the article explores how organizations in regulatory regimes respond, or are likely to respond, to multiple legitimacy and accountability claims, and how they themselves seek to build legitimacy in complex and dynamic situations. The arguments developed here are not normative: There is no ‘‘grand solution’’ proposed to the normative questions of when regulators should be considered legitimate or how to make them so. Rather, the article seeks to analyse the dynamics of legitimacy and accountability relationships as they occur in an attempt to build a more realistic foundation on which grander ‘‘how to’’ proposals can be built. For until we understand these dynamics, the grander, normative arguments risk being simply pipe dreams – diverting, but in the end making little difference. Keywords: accountability, legitimacy, polycentric regulation, regulation,…