When we talk about delusions we may refer to symptoms of mental health problems, such as clinical delusions in schizophrenia, or simply the beliefs that people cling to which are implausible and resistant to counterevidence; these can include anything from beliefs about the benefits of homeopathy to concerns about the threat of alien abduction. Why do people adopt delusional beliefs and why are they so reluctant to part with them? In Why Delusions Matter, Lisa Bortolotti explains what delusions really are and argues that, despite their negative reputation, they can also play a positive role in people's lives, imposing some meaning on adverse experiences and strengthening personal or social identities. In a clear and accessible style, Bortolotti contributes to the growing research on the philosophy of the cognitive sciences, offering a novel and nuanced view of delusions.