The Centers for Disease Control and Prevention-sponsored Autism and Developmental Disabilities Monitoring (ADDM) Network has monitored the prevalence of Autism Spectrum Disorders (ASD) among 8-year-olds in the US since 2000 using a rigorous record review methodology. This methodology has provided valuable insight into the epidemiology of the disorder, but recent studies that have employed screening and direct clinical assessment in the general population have reported significantly higher prevalence estimates than ADDM. The objectives of this study are to enhance our understanding of ADDM prevalence estimates, identify barriers to identification, and characterize the needs of children across the entire spectrum. This will be accomplished by conducting a screening and direct assessment study in the general population in an area already undergoing monitoring by ADDM. Specifically, all 8-year-olds in a three county region of South Carolina (n=8,000) will be screened for ASD using the Social Communication Questionnaire. All children identified as being at risk for an ASD, and 30% of those found to be at lower risk, will be invited to participate in an in-person diagnostic assessment (n=500). Case status will be assigned based on DSM-IV criteria and by scores on the Autism Diagnostic Observation Schedule and the Social Responsiveness Scale. Information regarding adaptive behaviors, level of impairment, cognitive skills, behavioral symptoms, and prior diagnoses will also be collected. Evidenced based strategies will be implemented to maximize participation in the screening and diagnostic phases of the study to improve accuracy of findings. ASD prevalence estimates will be calculated using the number of children aged 8 years residing in the study area as the denominator, and the number of children identified as cases as the numerator, with adjustments made for missing data from nonparticipants. Prevalence estimates will be provided using both traditional ADDM and ADDM Screening/Assessment methods. Factors contributing to differences in ASD prevalence estimates by methodology (e.g., sex, race/ethnicity, socioeconomic status, previous diagnoses, behaviors, degree of impairment, co-morbidities) will be examined. Because prevalence estimates are used to guide high-impact decision-making regarding critical issues such as allocation of funds for services and research, accurate reporting of ASD prevalence is important to researchers, health care providers, policy makers, and families.