Early diagnosis of autism spectrum disorder (ASD) facilitates access to interventions that improve outcomes for children and families. Understanding developmental and behavioral manifestations of symptoms and competencies improves early diagnosis. The authors illustrate the benefits of using the ZERO TO THREE "DC:0-5™: Diagnostic Classification of Mental Health and Developmental Disorders of Infancy and Early Childhood" (DC:0-5, 2016) multiaxial system for assessment of young children suspected of having a diagnosis of ASD. DC:0-5 provides a framework for forming rich conceptualizations that reflect the influence of the family system, physical health, psychosocial stressors, developmental competence and co-occurring mental health symptoms and disorders, to promote accurate early diagnosis and enhance treatment planning.