This chapter considers philosophical issues relating to the psychiatric diagnosis of autism. Autism is often considered a disorder of social understanding. Potential problems with this approach are highlighted, such as how theory of mind deficits may posit an unrealistic account of cognition and the social world. More sophisticated accounts of autistic cognition are outlined, but it is argued that these are compatible with the common assumption of an inbuilt lack of empathy. Theoretical explanations of autism are outlined and then assessed by considering philosophical notions of explanation. Each explanation of autism is in some sense limited, only explaining a particular domain and explaining it imprecisely, but this fact does not prevent them being legitimate explanations. Concerns that the heterogeneity of symptoms and causes of autism undermine the scientific status of the diagnosis of autism are outlined. Also, concerns that autism has been formulated in an arbitrary manner are outlined. These concerns can be countered by seeing autism as a scientific model that only imperfectly describes the world. This means it can be scientifically legitimate despite being heterogeneous. Notions of neurodiversity as fact and neurodiversity as a social movement are outlined. Whether neurodiversity is a fact, and in what manner it is a fact, depends upon what measurement system is employed. Neurodiversity as a social movement can help people with a diagnosis form social and cultural movements. There are dangers that social movements gloss over differences among individuals and that it may have limited representativeness of autistic individuals.