There is still debate as to what is the most effective strategy for identifying the early signs of autism in very young children. Two levels of screening having been advocated: broad-based developmental surveillance and targeted screening. Two popular tools for use in developmental surveillance are the Checklist for Autism in Toddlers (CHAT) and the Modified Checklist for Autism in Toddlers (M-CHAT). The purpose of this article is to summarise the current evidence for screening for autistic symptoms in very young children using CHAT and M-CHAT. A systematic search was carried out of electronic database and other sources for original studies which evaluated the use of CHAT and M-CHAT in screening for autism in children younger than 5 years of age. Studies were included for review if they evaluated the sensitivity and/or specificity of CHAT or M-CHAT, or described the best age to administer these instruments. The available evidence suggests that characteristic behaviours in autism should be evident in simple forms before the age of 18 months, while screening at 24 months should be conducted to identify those who regress. Administering a screening tool during 18- to 24-month well-child visits improves early identification of autism, while the stability of diagnosis at the ages of 18 months and 24 months is confirmed. M-CHAT has slightly better sensitivity and specificity compared to CHAT, and is preferable to use as a developmental surveillance screening instrument.