The Modified Checklist for Autism in Toddlers (M-CHAT) is one of the most widely used screening instruments for Autism Spectrum Disorder. The present chapter reviews the body of literature evaluating the validity of the M-CHAT and discusses recent evidence concerning the external validity of the measure in screening low-risk, general population, samples. Recent research showing that infants’ performance on social-cognitive experimental tasks provides support for the external validity of the M-CHAT as a parent-report measure. Specifically, expressive language, desire reasoning, and word learning abilities at 18 months of age have been found to concurrently predict scores on the M-CHAT. Furthermore, infants’ performance on tasks assessing social cognition and executive function at 10 and 12 months of age prospectively predicts scores on the M-CHAT, measured at 18 months of age. Future directions for the continued validation and potential modification of the M-CHAT are discussed.