Background: Progress has recently been made in the earlier identification of children with autism spectrum disorder (ASD). Whilst being welcome, this progress to earlier referral and diagnosis presents new challenges to clinical practice, including the accuracy and stability of early diagnosis, the utility of standardised assessment instruments with young pre-schoolers and the ability to indicate prognosis. Method: A selective review of recent research literature on the characteristic features of ASD in pre-school children. Results: Multidisciplinary diagnostic assessment should include detailed information on developmental history, parents' descriptions of the everyday behaviour and activities of the child, direct assessment of the child's social interaction style, including where possible with age peers, and formal assessment of communicative, intellectual and adaptive function. Clinical assessments need to concentrate on the identification of impairments in early non-verbal social communication behaviours that characterise children with ASD from the second year of life, including social orienting, joint attention, imitation, play and reciprocal affective behaviour. The particular pattern of symptoms that presents in a 2-year-old with ASD may differ from that seen at the more prototypic age of 4 or 5 years. In particular, overt repetitive and stereotyped behaviours may be less notable, although where these are seen alongside the social and communicative impairments they are highly indicative of ASD. The use of standardised assessment instruments and the strict application of the DSM and ICD diagnostic criteria need to be employed with caution, as an expert clinical view has been shown to be more accurate. An important aspect of early diagnostic consultation is an open and straightforward approach to the negotiation of the diagnostic view with parents over time. Conclusions: Earlier diagnosis and rising recognition of ASD have significant implications for primary healthcare and specialist diagnostic and therapeutic services.