Delayed sleep-wake phase disorder (DSWPD) is the most commonly encountered of the circadian rhythm sleep-wake disorders (CRSDs), and is often confused with sleep initiation insomnia. It typically emerges in teenage years and persists into adulthood. In essence, people with the disorder have an abnormally delayed major sleep episode relative to the dark phase of the solar cycle, and hence great difficulty initiating sleep at an appropriately early time, and, as a knock-on effect, waking at a desirable time in the morning, leading to chronic, and often quite severe sleep restriction trying to conform to a 9 to 5 schedule. As a result, sleep on free days is often extended in compensation. When released from such schedule constraints, sleep duration and quality is normal; it is just delayed. This review highlights elements of our current understanding of the epidemiology, associations and pathophysiology of the disorder, before discussing how some of our knowledge of sleep and circadian physiology can be applied to guide treatment of it.