The theory of mentalization describes the ability of individuals to make sense of their own and others’ behaviour in terms of mental states such as feelings, thoughts, beliefs and intentions. Mentalization is fundamental to people’s ability to manage relationships and emotions. Also, the process of mentalizing is core to the establishment of the therapeutic relationship and is a common factor in all therapeutic approaches. As such, mentalization is relevant to nurses and others working in a range of mental healthcare settings with any age group. This article introduces the theory of mentalization, including an explanation of its central principles, how individuals learn to mentalize during childhood, and how the ability to mentalize fluctuates in response to stress.