This chapter assesses the relationship of social anxiety disorder (SAD) to trait phenomena of shyness, behavioral inhibition, and avoidant personality. It explores the concept that these constructs lie on a spectrum of socially anxious temperaments, and that these temperaments are essential and adaptive for group-living species. Finally, it considers evidence that the same temperamental features influence, to a variable extent, a much broader spectrum of psychopathology. Several key features of SAD are also considered defining features of personality traits and disorders, including early age of onset, chronic course, pervasive quality, and tendency to be experienced as ego-syntonic. The spectrum of social anxiety temperament also appears to be related to a much broader spectrum of associated psychopathology. SAD has been reported to be a pre-morbid risk factor for schizophrenia and a comorbid disorder in as many as 36% of outpatients with schizophrenia.