Anxiety symptoms and disorders are highly prevalent in children and adolescents with Autism Spectrum Disorder (ASD), although they are often unrecognized or misdiagnosed. The purpose of the present review is to (1) provide clinicians with practical information on assessment and diagnosis of co-morbid anxiety in children and adolescents with ASD, (2) summarize and critically examine the literature on anxiety in children and adolescents with ASD, and (3) recommend avenues for future research in this area. A review of the literature yielded several recommendations for the assessment of anxiety in youth with ASD. It was concluded that comprehensive assessments of anxiety in ASD populations should use multiple informants, multimodal assessment techniques, and standardized assessment methods that are appropriate for clinical use in ASD samples. Overall, studies suggest that youth with ASD experience greater levels of anxiety than community populations, similar levels of anxiety to clinically anxious groups, and different patterns of anxiety when compared to other clinical groups. Although existing studies are methodologically fair, their correspondence with clinical recommendations for assessment is poor. Recommendations to improve of the quality of empirical studies and directions for future research are discussed.(Contains 4 tables.)