[Clin Psychol Sci Prac 17: 281–292, 2010] This article considers the nosology and pathogenesis of anxiety disorders in youth with autism. The comparability of anxiety in the autism spectrum disorder (ASD) population in relation to the typically developing population has been suggested by some recent findings, but conceptual and empirical ambiguities remain. It is suggested that anxiety may play at least three roles: (a) a downstream consequence of ASD symptoms (e.g., via stress generation through social rejection); (b) a moderator of ASD symptom severity, such that certain core autism symptoms like social skill deficits and repetitive behaviors may be exacerbated by anxiety; and (c) as a proxy of core ASD symptoms. Suggestions for clarifying the nature and function of anxiety in autism are made.