Theory of mind is the psychological capacity to attribute mental states to self and others. It appeared as a research tradition in the 1980s primarily through investigating the capacity for false belief understanding in preschool children. Three forms of explanation emerged in the developmental research that parallel related work in the philosophical literature: theory-theory, simulation theory, and modularity theory. More recently, infancy research relying on perceptual measures has been used to argue that false belief understanding emerges in the second or even first year of life. Although there is no strong evidence of false belief understanding in nonhuman species, including other great apes, comparative research has investigated theory of mind in other species for its own sake and also to give insight into its evolutionary history in humans. There has also been a long-standing interest in embedding theory of mind capacities in robotic agents as an embodied theory of artificial intelligence and to enhance human-computer interaction. With the recent advent of generative AI, there have also been discussions of whether large language models like ChatGPT should be thought to possess a theory of mind. Although the field has been the subject of sustained research for nearly 50 years, and has witnessed the extension of its concepts across the lifespan, to related psychological domains, to other species, and beyond the species boundary, there remains considerable debate about its core concepts, theories, methodologies, and whether theory of mind can be legitimately applied to infants, nonhuman animals, and artificially intelligent agents.