Advances in the identification of the early signs of autism spectrum disorder (ASD) have occurred despite the heterogeneity of the disorder and its variable onset and presentation. Using various methodologies including retrospective studies, community samples, and sibling cohorts, researchers have identified behavioral markers of the disorder that emerge over the first 2 years of life. However, there are characteristics of ASD that overlap with other types of developmental delay (DD), which may complicate differential diagnosis in young children. A review of the literature was conducted to identify the most promising behavioral markers that distinguish ASD from other types of DD in the first 2 years of life. The review identified profiles of behavioral markers in the social realm by 12 months and in the communication realm by 18 months, which along with additional atypical motor behaviors could distinguish ASD from DD. This constellation of features coupled with a flat or declining trajectory in specific aspects of social and communication development, may assist clinicians in targeting early interventions to at-risk infants. © 2013 Wiley Periodicals, Inc. Dev Disabil Res Rev 2011;17:130–140.