The dramatic increase in the detected prevalence of autism spectrum disorders (ASDs) in children over the past two decades is due, in large measure, to a broader case definition and more effective case finding. The similarities and differences in clinical presentation of ASDs and anxiety disorders have prompted great interest in anxiety in children with ASDs. In the absence of validated biological markers or diagnostic tests for anxiety disorders in this population, we rely on clinical observation, parent reports, and the child's verbal expression. The language and cognitive deficits in children with ASDs, however, hinder the application of assessments used in typically developing children. Because intervention research relies on reliable outcome measurement, better measurement of anxiety in children with ASDs is warranted.