This thesis presents the work completed over the course of the author’s doctorate of clinical psychology, and consists of two volumes. The first volume includes three chapters, the first of which is a systematic literature review and meta-analysis of the prevalence of insomnia in clinical and non-clinical populations of children and adolescents. The second chapter is an empirical paper exploring the relationship between sleep problems and daytime challenging behaviour in children with Autism Spectrum Disorders. The final chapter is a document summarising chapters one and two for the purpose of public dissemination. The second volume constitutes each of the five clinical practice reports completed over the course of the doctoral training and will be available online in December 2026.