The behavioral overlap between autism spectrum disorder (ASD) and intellectual disability (ID), as defined by the Individuals with Disabilities Education Act (IDEA), manifests in shared deficits in communication, social interaction, adaptive functioning, and educational performance. These overlapping behaviors—such as delayed language development, limited peer engagement, and challenges with daily living skills—are evident across age groups from 0-12 years. This convergence arises from shared neurodevelopmental mechanisms, overlapping diagnostic criteria, developmental delays, environmental influences, and high comorbidity rates. Understanding these factors is essential for accurate diagnosis, effective interventions, and tailored educational support. In this resource, I explore the reasons for this overlap, grounded in clinical research, neuroscientific insights, and the behavioral patterns previously outlined.