Service users’ involvement in mental health service research is increasingly acknowledged as important, yet, whilst involving users of mental health services as research participants is commonplace, seeking out their experience and indeed their “expertise” to facilitate the development of tools to be used within mental health services is in its infancy. This article describes the involvement and views of service users in the development of a nursing metric—the Therapeutic Engagement Questionnaire. It presents their role in the three stages of development: generation, statement reduction and authentication.