Autism spectrum disorder (ASD) is a neurodevelopmental disorder, multi‐factorial in origin, often presenting in early childhood. It is characterized by impairments in social‐communication; restricted/repetitive patterns of behaviours, interests and activities, and by delays in adaptive functioning. The objective of this chapter is to review, using mostly systematic reviews and meta‐analyses, the clinical presentation, aetiology, assessment and treatment of ASD. Advances in recent years have emphasized the heterogeneity of presentation and underlying pathophysiology. Finding the most appropriate interventions for the particular needs of the developing child with ASD requires an understanding of individual differences at multiple levels. A greater awareness of this heterogeneity is beginning to emerge which should advance our understanding of ASD and inform the design of appropriate intervention research.