Autism spectrum disorder (ASD) is characterized by deficits in social communication and social interaction together with restrictive, repetitive, and inflexible patterns of behavior including routines, rituals, and overfocused interests, sensory sensitivities, and motor stereotypies (e.g., hand flapping when excited or anxious). ASD is classified in the 11th revision of the International Classification of Diseases (ICD-11) as a neurodevelopmental disorder because the condition results from dysregulation of the brain's development. A different pattern of differential diagnoses is found at each developmental stage. The distinction from disorders of intellectual development, developmental speech or language disorders, and hearing impairment is particularly relevant during the preschool period. Preliminary evaluation of suspected ASD can be done in a primary care setting, but a full diagnostic workup requires specialist assessment. Apparent rates of ASD vary among cultures, but these are likely to reflect differences in recognition and diagnosis rather than underlying population characteristics.