In recent years, the legitimacy of corporations has been increasingly challenged by civil society actors, especially in public debates about sustainability. This chapter sheds light on strategic communication of civil society actors that strives to delegitimate corporations engaging in unsustainable practices or displaying (potential) discrepancies between their corporate activities and communication on sustainability efforts. The chapter discusses the characteristics of such communication, particularly its strategies and processes. Based on a theoretical literature review, the chapter focuses on public relations studies that are complemented by literature from neighboring fields, such as organizational and environmental research. It contributes to expanding knowledge on the delegitimation of others in public debates about sustainability through strategic communication (hereon in “delegitimating strategic sustainability communication”)—a topic that is of great importance in current debates on sustainability but has so far been treated with restraint—and offers impulses for further research.