Background: It is a truism that nursing care must be informed by assessment, otherwise how can one know what care is required or that it has been successfully delivered? Yet, little is known about the process of comprehensive mental health nursing assessment in practice. If the education of mental health nurses is to be effective, it is essential that the key content of, and the processes involved in carrying out a mental health nursing assessment in practice are able to be articulated to learners. Aim: To identify the processes of assessment that occur in mental health nursing practice based on interviews with mental health nurses working in clinical and management roles in clinical areas. Method: Interviews were undertaken with 18 nurses who worked in inpatient and community mental health settings either as clinicians or managers. The nurses ranged from new graduates to those with more than 20 years of experience. Findings and discussion: Clear processes were reported to be involved in undertaking a comprehensive mental health nursing assessment in practice, with three main themes emerging during analysis. First is the importance of engaging the patient; second is tell me what the problem is? with one subtheme reconcile inconsistencies; and finally, the ongoing nature of the assessment process. Conclusion: Common processes emerged when the nurses described their individual approaches to undertaking comprehensive mental health assessment. The results have important policy implications for the educational preparation of mental health nurses, their ongoing supervision and further research into contemporary mental health nursing practice.