This chapter investigates the theoretical and practical intersections of mediatization theory, the “logics” approach to public diplomacy, and the concept of epistemic security. Mediatization theory challenges our understanding of diplomacy by showing how diplomacy is not merely communicated through media but increasingly constituted by media logic. The “logics” approach expands on this by introducing a pluralistic, institutionalist framework that explains the dynamic rationales underpinning different public diplomacy practices. This chapter examines these theories against the backdrop of what might be termed a crisis of epistemic security; that is to say, significant challenges to what constitutes valid knowledge in foreign policy. Using three brief empirical cases, the chapter illustrates how the mediatization of diplomacy and its guiding logics both shape and are shaped by the increasing instability of shared knowledge frameworks in the post-truth era. The essay concludes that promoting and safeguarding epistemic security is now a central task for diplomacy, and that mediatization and logics theories offer valuable theoretical tools to understand and navigate this challenge.