Delusions are easier to characterise than to define. A deluded individual has a belief (or beliefs) that is held with a degree of certainty unwarranted by the evidence available to that person, that is strongly resistant to vigorous opposition from others, and that distresses or preoccupies the person or otherwise disrupts their everyday functioning. While some aspects of this characterisation (for example, the notion that delusions are beliefs) have been subject to philosophical debate, this entry will focus on issues of classification, aetiology and treatment.