The American Academy of Pediatrics recommends that all children be screened with an autism spectrum disorder (ASD)-specific screening tool at 18- and 24- months. However, there currently is not a well-validated ASD-specific screener for use at 18-24 months in pediatric settings, making the need for validated screening tools at these young ages urgent. This research project is designed to study the following screening and evaluation measures: 1) the Early Screening for Autism and Communication Disorders (ESAC), a new autism-specific parent report screening tool for children 12-36 months; 2) the Systematic Observation of Red Flags of ASD for a clinical setting (SORF-Clinic), 3) the Systematic Observation of Red Flags of ASD for a home setting (SORF-Home), and the Autism Diagnostic Observation Schedule (ADOS) Toddler Module newly developed for children under 30 months of age. A total of 600 children from 18-24 months of age will participate in this study. A best estimate clinical diagnosis will be made as the gold standard to compare the other tools. The expected outcomes of this study will have significance for the field by documenting the accuracy of a set of promising screening and evaluation measures for toddlers that maximizes the role of families to improve early detection of ASD. The comparison of autism symptoms displayed at home in everyday activities with those displayed in clinical settings will have important implications for building consensus with families of toddlers about early red flags of ASD and the urgency of early diagnosis and for developing more effective screening methods. These findings will advance science by providing researchers methods for identifying and hence studying children with ASD at younger ages, which could impact genetic, biomedical, and intervention research. By improving and streamlining early screening and diagnosis of ASD in 18 to 24 month old children, the findings of this study will have important implications for earlier access to intervention.