 This entry can only address some of the philosophical debates centered on delusions. Section 1 provides an overview of the philosophical significance of delusions. Section 2 introduces the issues surrounding the controversial definition of delusions, and explains some of the common distinctions between types of delusions. Section 3 discusses the most prominent theoretical approaches to the nature and formation of delusions and the conceptual questions emerging from such approaches. Section 4 reviews some of the most discussed themes in the philosophical literature on delusions: whether delusions are irrational; whether they are beliefs; and to what extent they overlap with cases of non-clinical beliefs such as self-deception and beliefs in conspiracy theories.