This article reviews evidence-based criteria that can guide practitioners in the selection, use, and interpretation of assessment tools for autism spectrum disorders (ASD). As Mash and Hunsley (2005) discuss in this special section, evidence-based assessment tools not only demonstrate adequate psychometric qualities, but also have relevance to the delivery of services to individuals with the disorder (see also Hayes, Nelson, & Jarrett, 1987). Thus, we use what is known about the symptoms, etiologies, developmental course, and outcome of ASD to evaluate the utility of particular assessment strategies and instruments for diagnosis, treatment planning and monitoring, and evaluation of outcome. The article begins with a review of relevant research on ASD. Next we provide an overview of the assessment process and some important issues that must be considered. We then describe the components of a core (minimum) assessment battery, followed by additional domains that might be considered in a more comprehensive assessment. Domains covered include core autism symptomatology, intelligence, language, adaptive behavior, neuropsychological functions, comorbid psychiatric illnesses, and contextual factors (e.g., parent well-being, family functioning, quality of life). We end with a discussion of how well the extant literature meets criteria for evidence-based assessments.