Theory of mind (ToM) is key to the development of one's social skills. Without ToM, children (or adults) cannot understand or infer the thoughts, feelings, or intentions of others. A lack of ToM skills is considered by some to be a core deficit in autism. ToM affects all interpersonal interactions as well as academics, daily living, following directions, and understanding socially-based information. For children with autism spectrum disorder (ASD), problems such as emotion recognition and sharing, understanding knowledge, deception, humor, teasing, and describing one's mental state or feelings are related to ToM impairment. Although there are few interventions targeting ToM skills, there is some research evidence that these skills can be taught, at least to a limited extent. Due to the complexities of the social world and ToM development, it has been difficult to produce observable social change through short-term intervention. Yet as more is learned about ToM and the range of social skills influenced by it, one can expect the development of more successful, evidence-based interventions. Practical implications from research on ToM are discussed. ["Unlocking the Mystery of Social Deficits in Autism: Theory of Mind as Key" was written with Erica Howell.]