The research described in this thesis investigated colour processing in children and adolescents with autism spectrum disorders. Although idiosyncratic responses to colour have been widely reported in autism (William, 1999; White & White, 1991), and therapeutic interventions involving colour are frequently used with individuals with this disorder (Howlin, 1996; Irlen, 1991), few controlled colour processing investigations have been carried out. The experiments reported in the thesis have two main points of focus. Firstly, the therapeutic effects of colour overlays on different aspects of cognition were tested, and secondly, studies into colour discrimination, memory, naming and categorisation were carried out in order to evaluate the role of language and perceptual processing in colour processing. In experiments one and two it was established that significantly more children with autism than age and intelligence matched controls improved their reading speed when using a colour overlay. In experiments three and four, these effects were further investigated using visual change detection and reading comprehension tasks with and without colour overlays. Again, a significant improvement in performance was noted in the autism group when using colour overlays. The results from experiments four to eleven, testing colour discrimination, memory and naming failed to confirm atypical colour processing in autism, although the findings did suggest that cognitively unimpaired children with autism showed sharper category boundaries than those with autism and cognitive impairment and typically developing controls. Finally data from a case study of a boy with Asperger Syndrome who showed highly idiosyncratic colour responses were presented. The findings from the studies are discussed within the context of current theories of visual cognition in autism and theories of colour perception.