Anxiety is common in children and adolescents with autism spectrum disorders (ASD), and may worsen during adolescence. This review summarizes research on the prevalence, phenomenology, and treatment of anxiety in youth with ASD and related conditions. A systematic search of 40 studies published between 1990 and 2008 found that anxiety is prevalent in ASD, with rates ranging from 11% to 84%. Anxiety disorders such as simple phobias, generalized anxiety disorder, separation anxiety disorder, obsessive-compulsive disorder, and social phobia are frequently reported. Anxiety is more common in children with Asperger's Disorder than in those with Autistic Disorder. Anxiety symptoms are often not age-typical in ASD and may be more associated with acting out behaviors than in non-ASD children. Anxiety may have bidirectional effects with the social disabilities of ASD. The assessment of anxiety disorders in ASD should be conducted using multiple informants and modalities. There is a need for more controlled intervention studies to evaluate the efficacy of pharmacological and psychosocial approaches. Cognitive-behavioral interventions show promise as an effective treatment modality for co-occurring anxiety in ASD, but most studies have focused on higher functioning children. The review highlights the importance of understanding the development of anxiety and its interaction with the core disabilities of ASD. It also emphasizes the need for further research on the neurobiological bases of comorbid conditions in ASD to distinguish truly clinically distinct comorbid disorders from features associated with ASD itself. The results suggest that anxiety is common in ASD and may be a source of additional morbidity. The review concludes that anxiety is a significant concern in ASD and that further research is needed to develop effective assessment and treatment approaches.Anxiety is common in children and adolescents with autism spectrum disorders (ASD), and may worsen during adolescence. This review summarizes research on the prevalence, phenomenology, and treatment of anxiety in youth with ASD and related conditions. A systematic search of 40 studies published between 1990 and 2008 found that anxiety is prevalent in ASD, with rates ranging from 11% to 84%. Anxiety disorders such as simple phobias, generalized anxiety disorder, separation anxiety disorder, obsessive-compulsive disorder, and social phobia are frequently reported. Anxiety is more common in children with Asperger's Disorder than in those with Autistic Disorder. Anxiety symptoms are often not age-typical in ASD and may be more associated with acting out behaviors than in non-ASD children. Anxiety may have bidirectional effects with the social disabilities of ASD. The assessment of anxiety disorders in ASD should be conducted using multiple informants and modalities. There is a need for more controlled intervention studies to evaluate the efficacy of pharmacological and psychosocial approaches. Cognitive-behavioral interventions show promise as an effective treatment modality for co-occurring anxiety in ASD, but most studies have focused on higher functioning children. The review highlights the importance of understanding the development of anxiety and its interaction with the core disabilities of ASD. It also emphasizes the need for further research on the neurobiological bases of comorbid conditions in ASD to distinguish truly clinically distinct comorbid disorders from features associated with ASD itself. The results suggest that anxiety is common in ASD and may be a source of additional morbidity. The review concludes that anxiety is a significant concern in ASD and that further research is needed to develop effective assessment and treatment approaches.