This chapter gives an illustrated overview of recent philosophical work on the concept of delusion. Drawing on a number of case vignettes, examples are given of the wide range of theories that has been advanced to explain this most challenging of experiences. Some have agreed with the philosophical founder of modern descriptive psychopathology, Karl Jaspers, that delusions are (empathically at least) "ununderstandable." The large majority, though, has sought to understand delusion in terms of aberrations of one kind or another either of beliefs (or related mental contents such as imaginings) or of the grounds or preconditions for beliefs. As a project in understanding, these theories offer helpful insights. A