Research indicates that approximately one in six people will experience a common mental health condition such as depression or anxiety, while people with severe mental illness commonly experience significant health inequalities. Nurses will come into frequent contact with people experiencing mental health issues in hospitals and in the community. Mental state assessment is a tool that enables nurses to develop an understanding of an individual’s mental health that will be universally recognised across healthcare services. This article provides an outline of the various elements involved in a mental state assessment, so that clinicians such as non-mental health nurses can identify people experiencing deteriorating mental health.