A substantial number of adults suffer young-onset dementia (YOD). The diversity of types and syndromes makes recognition and diagnosis difficult. An algorithmic approach to interpreting clinical data, informed by clinical epidemiology, integrates data pertaining to defining syndromes. Syndrome chronology and tempo, family history, and other neuropsychiatric features and neurologic signs, are used to reach a preliminary diagnosis and direct diagnostic tests and their interpretation. Screening for YOD in the psychiatric context is a rational process in which vigilance is combined with careful searches for red flags that signal a neurodegenerative etiology