The autism spectrum is a range of neurodevelopmental conditions generally characterized by difficulties in social interactions and communication, repetitive behaviors, intense interests, and unusual responses to sensory stimuli. It is commonly referred to as autism or, in the context of a professional diagnosis, autism spectrum disorder (ASD).

A spectrum disorder is one that can manifest very differently from person to person: any given person with the disorder is likely to show some but not all of the characteristics associated with it, and may show them to very different degrees. Different autistic people might show strikingly different characteristics, and the same person might also present differently at different times. Historically, the autism spectrum was divided into sub-categories, but questions persisted over the validity of these divisions. The most recent editions of the major English-language diagnostic manuals, Diagnostic and Statistical Manual of Mental Disorders (DSM-5-TR, published in 2022) and International Classification of Diseases (ICD-11, released in 2021) both list ASD as a single disorder.

While psychiatry traditionally classifies autism as a neurodevelopmental disorder, many autistic people, most autistic advocates and a rapidly increasing number of researchers see autism as part of neurodiversity, the natural diversity in human thinking, and experience, with strengths, differences, and weaknesses. On this view, promoted by the autism rights movement, autism is not pathological, but this does not preclude autistic individuals from being disabled and potentially having high support needs due to co-occurring conditions and lack of person-environment fit. This relatively positive and holistic view of autism has led to a certain degree of friction between autistic individuals, advocates, charities, researchers and practitioners.

Other controversies in autism are scientific, sociological, political, or philosophical, and some have aspects of all four. First, it is controversial and uncertain if social-communication difficulties of autistic people are inherent core deficits (see empathizing-systemizing theory developed by Simon Baron-Cohen), or due to mismatch in social communication styles, cognition, and experiences resulting in bidirectional misunderstanding between autistic people and non-autistic people (see double empathy problem theory developed by autistic researcher Damien Milton and recent growing evidence that found that autistic people empathize, communicate and socialize well with autistic people), or a combination of both factors. A 2018 study has shown that autistic people are more prone to object personification, suggesting that autistic empathy may be not only more complex but also more all-encompassing, contrary to the popular belief that autistic people lack empathy.

Scientists are still trying to determine what causes autism; it is highly heritable and believed to be mainly genetic, but there are many genes involved, and environmental factors may also be relevant. There is also no cure for autism, so interventions focus on, for example, finding and learning other modes of communication in a non-verbal autistic person, or applied behavior analysis interventions. It is unclear why autism commonly co-occurs with ADHD, epilepsy and a range of other conditions. There are ongoing disagreements about what should be included as part of the autism spectrum, whether meaningful sub-types of autism exist, and the significance of autism-associated traits in the wider population. The combination of broader criteria and increased awareness has led to a trend of steadily increasing estimates of autism prevalence, causing a common misconception that there is an autism epidemic and perpetuating the myth that it is caused by vaccines.