This chapter discusses institutional theory and its application to sustainability accounting. The chapter is divided into two main sections. The first explores the concept of institutional pressures and how these pressures influence the behaviour of organizations, often resulting in similar procedures being adopted by organizations under similar pressures, for example publishing sustainability and integrated reports that adopt common structures and disclose similar information. The second section discusses the concept of institutional work, which acknowledges that social actors (for example managers and professionals) are able to create new institutions, and maintain or disrupt existing institutions.