This article provides an introduction to seasonal affective disorder (SAD) and outlines various therapies, including phototherapy (light therapy), used in its treatment. SAD, colloquially termed 'winter blues', is a common condition that is thought to be caused by reduced levels of daylight in winter. During this period sufferers generally feel low and may experience clinical depression. The Department of Psychiatry at the University of Southampton has an established SAD service as part of its mood disorders clinic, which was developed from a research-based clinical investigation unit set up in the early 1990s. SAD is described as a mood disorder with a seasonal pattern and has a greater prevalence in countries with greater northern latitude. The aetiology of SAD is unclear, although the most promising theory suggests the role of the neurotransmitter serotonin. SAD is difficult to treat with conventional antidepressants although there is evidence that serotonin selective reuptake inhibitors may be useful for some patients. Phototherapy (light therapy) has been used successfully by many patients although it remains controversial and difficult to obtain on the NHS.