Delusional disorder, rare in clinical practice, remains one of the most enigmatic conditions in psychiatry. Linked to schizophrenia spectrum disorders since the mid-20th century, recent work has confirmed 19th-century notions of an aetiology based on attentional biases and personality dimensions. Unfortunately, the literature exists largely as case reports and series, often published as ‘curiosities’, and therefore lacks the rigour of formal diagnosis. This article reviews current thinking on aetiology and epidemiology, considers diagnosis, and reviews recent work on physical and psychological therapies. It concludes that delusional disorder is likely to respond well to treatment with standard antipsychotics, often at low doses, but that adherence and concordance remain particular problems. Cognitive therapy has been shown to be beneficial but is time-consuming and resource-intensive. Selective serotonin reuptake inhibitors may profit further investigation. There remains considerable scope for investigation of this fascinating condition.