This chapter provides a brief outline of legitimacy theory. It draws a distinction between an institutional and strategic perspective on organizational legitimacy. The latter is used to examine how companies rely on their sustainability or integrated reports to maintain and repair legitimacy. Four broad strategies are used for this purpose: avoidance, altering values and expectations, managing perceptions and adaption and conformance. Companies may rely on one or more of these strategies to manage their legitimacy depending on whether they are trying to gain, maintain or repair legitimacy and the significance of legitimacy for securing stakeholders’ support.