Aims and objectives.To develop an integrated cognitive and social understanding of assessment in mental health nursing.Background.Assessment is a vital component of nursing care for mental health service users, largely driven by a tacit, experiential model of assessment; this approach is at variance with an evidence-based approach to assessment.Design.A qualitative design was employed in the study, with a thematic analysis carried out on transcripts of focus groups with mental health nurses.Method.Ten focus groups were carried out, guided by questions on nurses' contribution to care and the problems patients present with. Fifty-nine registered mental health nurses were sampled from eight acute and community mental health services across urban and rural regions in Ireland.Results.References to assessment were identified (how nurses acquired information, how it was made sense of and used in the system of care). Assessment talk was characterised by reliance on a experientially based clinical schema and recognition of the task environment's shaping influence. Nurses' clinical knowledge was a pragmatic tool that permitted nurses to assess risk, promote patient engagement and work with doctors.Conclusions.Nurses strived to 'know the patient', while having to 'work the system', with implications for patient care and decision-making quality. Reliance on experiential knowledge is a professional trait, but one that renders nursing assessment 'invisible' in significant ways.Relevance to clinical practice.Cognitive and social aspects of nursing decision-making have been considered apart from one another, whereas cognitions about mental health conditions are, in fact, applied in a pragmatic, task-oriented organisational system. Nurses believed that spending time with the service user led to a privileged position of knowledge in comparison with doctors ('knowing the person'), but this knowledge is frequently applied to the task of 'knowing the patient', assessing the person as a source of risk and danger.