This paper presents a comprehensive review of delayed sleep phase syndrome (DSPS), a circadian-rhythm sleep disorder thought to result from the endogenous circadian pacemaker being "stuck" at a later-than-normal phase, relative to the desired sleep-wake schedule. A full understanding of this disorder is best appreciated from the context of shared modulation of sleep and wakefulness via sleep homeostatic and circadian systems. Typically emerging during adolescence, DSPS comes to clinical attention much less often than prevalence estimates would suggest, perhaps due to underrecognition by clinicians and misattribution of symptoms. Several treatment modalities have been suggested, including phototherapy, chronotherapy, and exogenous melatonin administration. However, caution is raised for the reason that more than 20 years after its initial description in the literature, the basic pathophysiology of DSPS remains poorly understood, as observed in the 2003 National Sleep Disorders Research Plan. Challenges for future research include elucidating the exact sleep homeostatic and circadian contributions to the disorder, improving the objective verification of this diagnosis instead of relying only on self-report information, and conducting treatment research aimed at determining efficacy, effectiveness, and mechanism or mechanisms of action.