A meta-analysis was conducted to evaluate the effects of early behavioural intervention for children with autism. A review of the literature identified nine 3 controlled studies that met criteria for inclusion. The results of the analysiS yielded an average effect size of1.1 0 for gain in 10, and .66 for adaptive behaviour gains. These effect sizes would generally be considered as large and moderate, respectively. Following on this an individual participant data meta-analysis was done. Here, participants from 16 studies were divided into three groups depending on the type of intervention they had received; 309 children had received behavioural intervention, 39 comparison interventions, and 105 were in a control group. Results showed that more children receiving behavioural intervention achieved reliable change in 10, and adaptive behaviour. This equated to a Number Needed to Treat of 5 for 10 and 7 for adaptive behaviour. Regression analysis showed of this larger sample showed that 10 and adaptive behaviour at intake predicted gains in adaptive behaviour and that intensity of intervention predicted gains in both 10 and adaptive behaviour. Next, I evaluated the utility of behavioural intervention implemented in local mainstream preschools. Outcome, particularly on 10, appeared satisfactory when compared against suggested benchmarks, even though it proved difficult to achieve the recommended intensity of intervention. Finally, I evaluated if behavioural intervention could benefit children with intellectual disabilities. A group of children (n=11) with intellectual disabilities received approximately 10 hours per week of behavioural intervention and another group (n=14) received treatment as usual. After one year, changes in intelligence and adaptive behaviour were significantly in favour of the behavioural intervention group. In general my 1 results support the clinical implication that at present behavioural intervention should be the intervention of choice for children with autism, and also appears promising for children with intellectual disabilities.