Theory of Mind (ToM) is the ability to think about what others see, know, think, want and intend, and is thought to be a fundamental basis of social interaction and communication. ToM has been widely studied in young children and infants, and more recently its cognitive and neural basis has begun to be studied in adults. ToM-use is an important question for understanding our social interactions and language, where ToM abilities are frequently believed to be critical, and where there is evidence of significant variation in ToM-use both between individuals and between "Western" and "Eastern" cultures. Understanding when and how ToM is used is also important for studying differences in ToM following brain injury, or in people with autism. Finally, with increasing "socialisation" of human-computer interfaces, which may use virtual agents who speak and interact with their users, it is ever more important to know when and how humans succeed and fail in their use of ToM. Only by doing so will it be possible to optimise such systems for their human users. The current project will investigate fundamental questions about ToM-use that will be of interest to these broad audiences.