Seasonal fluctuations in mood, drive, energy, sleeping- and eating behavior, weight, as well as further important mental and physical functions, and the utilization of light as an effective treatment option were already described by Hippocrates of Kos and Araeteus, the Cappadocian. The concept of the so-called seasonal affective disorder (SAD) as a disruption of the circadian rhythm precipitated by a deficiency of environmental light during darker seasons was first described in the 1980s. Furthermore, chronobiological and hormonal dysregulation in SAD patients was repeatedly shown to be accompanied by alterations on a neuroreceptor and neurotransmitter level and to normalize after remission. Hence, SAD represents one of the most important models of a chronobiological disorder with over 1000 international publications on its aetiology and treatment options, whereby their underpinnings could be elucidated on a clinical as well as molecular level. The present article summarizes the current understanding of etiological mechanisms of SAD and provides an overview of diagnostic and therapeutic strategies, which are based on available international evidence including clinical trials, systematic reviews, and meta-analyses. According to current recommendations of international guidelines, promising treatment options as bright light therapy, psychopharmacotherapy, therapeutic sleep deprivation, and their underlying mechanisms of action are presented.