Early intervention for autism spectrum disorders necessitates early detection. This need has led to widespread agreement across disciplines that screening is critical in very young children. Two screening issues are highlighted in this review. Level of screening refers to the type of sample: Level I is defined as an unselected sample, and Level II consists of selected children already identified as being at risk for a developmental disorder. Breadth or scope of screening refers to the range of difficulties the screening tool attempts to identify: broad screening instruments identify multiple range of developmental difficulties, whereas disorder-specific tools focus on a single disorder or class of disorders. Broad developmental instruments reviewed include the Parents' Evaluation of Developmental Status and the Ages and Stages Questionnaires; autism-specific tools reviewed include the Checklist for Autism in Toddlers, the Modified Checklist for Autism in Toddlers (M-CHAT), the Pervasive Developmental Disorders Screening Test, Second Edition, and the Screening Tool for Autism in Two-year-olds. The development of the M-CHAT, a Level I and Level II screening instrument, is described, and current research and clinical use of the M-CHAT are reviewed, including description of the structured follow-up interview which reduces the false-positive rate of the parent-report M-CHAT.