This article presents a three-stage model of how isomorphic mechanisms have shaped corporate social responsibility (CSR) reporting practices over time. In the first stage, defensive reporting, companies fail to meet stakeholder expectations due to a deficiency in firm performance. In this stage, the decision to report is driven by coercive isomorphism as firms sense pressure to close the expectational gap. In the second stage, proactive reporting, knowledge of CSR reporting spreads and the practice of CSR reporting becomes normatively sanctioned. In this stage, normative isomorphism leads other organizations to look to CSR reporting as a potential new opportunity for achieving the firm’s goals. In the third stage, imitative diffusion, the defensive reporters together with the proactive reporters create a critical mass of CSR reporters that reaches a threshold at which the benefits of CSR reporting begin to outweigh any costs due to mimetic isomorphism. The study finds support for the model in an examination of Fortune 500 firms from 1997 to 2006.