We will review typical social development in early childhood, the basics of the Autism Diagnostic Observation Schedule-2 (ADOS-2), and the multidisciplinary team model of Autism Spectrum Disorder (ASD) assessment. To diagnose or rule out ASD for toddlers and preschoolers, clinicians should be familiar with typical developmental milestones, particularly social development. Understanding typical social development is very important in discerning atypical, delayed, or absent social behaviors. The ADOS-2 is often regarded as a reference standard in the ASD field. Basic knowledge of the ADOS-2 will guide clinicians in interpreting its findings. Early childhood development is a dynamic process involving multiple domains of cognition, language, motor, play, and social. To assess ASD concerns, clinicians should be able to assess each developmental domain and integrate all developmental information. The ideal is a multidisciplinary team, as will be described below. In the absence of a multidisciplinary team, or a clinician trained in the ADOS-2, a pediatrician or child psychiatrist can be armed with screening tools, and an understanding of development and ASD to make a diagnosis early, and assist in access to early intervention.