Hospitals and emergency departments (EDs) are caring for increasing numbers of patients who present with underlying mental health issues. Management of these patients can be challenging for clinical staff who often lack the specialist knowledge and skills required to provide appropriate care. In April 2015, a mental health liaison service was introduced in Rotherham Hospital as part of a two-year pilot scheme. The aim was to provide assessment and treatment of adults with mental health problems admitted to the hospital. An evaluation of the new service was undertaken to assess the effects of the service. This article, part one of two, reviews the literature, as well as outlining the background to, and method of, evaluation. Part two, which will be published in December, examines the themes revealed by analysis of the service participants’ responses.