BACKGROUND: Insomnia is a common condition affecting individuals of various ages. It is diagnosed on the basis of a self-reported complaint of poor sleep quality concomitant with daytime disturbances. If left untreated, insomnia is associated with a number of adverse health outcomes. OBJECTIVE: The aim of this article is to review key diagnostic criteria, theories and assessment of insomnia. DISCUSSION: Insomnia may be precipitated by stressful events. Unhelpful strategies employed by the individual to remedy sleep can perpetuate insomnia symptoms even after the stressful event subsides. Insomnia is often undiagnosed and undertreated, which is concerning given that untreated insomnia associated with a number of negative health outcomes. Diagnosis and treatment planning can be facilitated using self-report measures.