This chapter provides an overview of early milestones for social development and discusses the markers for social disabilities, such as autism spectrum disorder, that can be identified in children at risk. The principles and elements of a comprehensive clinical evaluation of infants and toddlers at risk for autism spectrum disorder will be discussed, including how to differentiate social disabilities from non–autism spectrum disorder developmental disorders. While some of these principles can build on proven experience in the assessment of older children (Klin, Carter, & Sparrow, 1997), there is an emphasis on unique challenges and opportunities associated with our attempt to intuit, measure, and explain social development at these tender ages (Chawarska, Klin, Paul, Macari, & Volkmar, 2009; Macari et al., 2012; Zwaigenbaum et al., 2009).