This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book considers autism at different levels of explanation: biological, cognitive and behavioural. It provides an overview of how the cognitive level may be used to link behavioural and biological features. The book reviews criteria for a good psychological theory, considering quality and quantity of evidence and practical impact. Developmental progress models tend to frame autism as the result of multiple interacting processes across early development. Cognitive difference models are concerned with the complex ways in which people with autism relate to their environments and attempt to characterise autism on that basis. Cognitive theories aim to span the gulf between biology and behaviour through hypotheses about the mind. The book also reviews the contribution that psychological theory has made to our understanding of autism and to practice in schools, clinics and beyond.