The Child Behavior Checklist (CBCL) [5, 56]. This tool is part of the Achenbach System of Empirically Based Assessment. It is a widely used tool, with two formats for children at different age bands. This is a particular strength for longitudinal studies, and both versions are included in the review. The 1.5–5-year format has 99 items , and the 6–18 years version has 118 items, with norms available for typically developing children. The items can be scored on psychiatric scales related to DSM, though this may not be relevant for children with ASD up to 6 years. It was used in three observational [ 19, 24, 43 ] and three [ 32, 41, 42 ] intervention evaluation studies in this review.   The CBCL was one of the two behaviour questionnaires presented to participants at the Discussion Day. They liked the clear instructions, with a time frame of two months, and the wide range of questions, including a qualitative section at the end enquiring about the best things about the child. However,