For more than three decades, theory of mind (ToM) has been one of the leading and prevalent issues in developmental psychology. ToM is the ability to ascribe mental states (e.g. beliefs, intents, desires, pretending, knowledge) to oneself and others as well as to recognise that others have beliefs, desires, and intentions that differ from one's own. The purpose of this article is to provide a background on ToM's theoretical origins, describe young children's understanding of mental states, explain the use of false beliefs tasks as an assessment measure, report several false belief studies and their age-related variations, describe the children's mental models, and summarise the children's understanding of mental states.