The Intelligence chapter initially explores the relationship between Intelligence Quotient (IQ) and Autism Spectrum Disorders (ASD). It introduces recent developmental and intelligence tests used in Japan, organized by developmental stages, and emphasizes the importance of ASD assessments. Additionally, the chapter investigates differences in traits between children and adults through studies on Intelligence Quotient in individuals with ASD. Next parts of the Intelligence chapter explore the complex interrelationships between ASD, Intellectual Disability (ID), and cognitive functions. It provides an in-depth analysis of recent research methods, neurological science advances, genetic and environmental factors, and the impact of early social experiences on cognitive development. The text discusses the co-occurrence of ASD and ID, highlighting their shared and distinct etiological factors, the influence of sensory processing disorders, and the significance of early detection and intervention. It also examines genetic and epigenetic research contributions to understanding ASD and ID, emphasizing the need for comprehensive therapeutic approaches and educational placements for affected individuals. Last parts of the Intelligence chapter explore the association between neural function/morphology and intellectual ability in people with ASD. Exiting EEG/MEG studies indicated a potential link between ID and dysregulation of cortical excitability in children with ASD. Functional connectivity studies raised the possibility that differential neural networks contribute to intellectual function between individuals with and without ASD. However, the number of relevant studies is relatively small, and as such considerable effort is required to elucidate the neurophysiological basis of intellectual function in people with ASD.