The overarching aim of this review is to provide a rapid appraisal of published, international peer-reviewed mental health academic papers and UK policy focused on safe staffing in relation to both in-patient and community mental health services. The remit is to build on the previous Mental Health Services evidence review (Lawes et al. 2018). The commissioning brief set out two areas of interest. The first is to explore the evidence around skill-mix across mental health services focusing on the addition and contribution of other roles and the relationship to patient outcomes. We intend this to be laser-focused on mental health nurses, the largest professional body within mental health services. This will include the skill mix of mental health nurses within nursing teams and across mental health services. A recent review sought to contextualise skill mix as having three dimensions (Cunningham et al. 2019). Firstly, mental health nurse role and function, i.e. skills, abilities, competencies, and knowledge). Secondly, intra-professional transversality of practice, i.e. grade, ratios of nursing staff, level of qualifications, expertise, experience, education and training. Finally, inter-professional transversality of practice, i.e. ratios of mental health nurses in multi-disciplinary teams. A review conducted by the National Institute for Health and Care Excellence (Rutter et al. 2015) for the Department of Health and NHS England found low quality evidence across 10 studies of an association between inpatient mental health nurse staffing levels and a range of outcomes including conflict and containment rates. Additionally, Moyo et al. (2020) investigated the association between the registered mental health nurse-to-registered nurse ratio and psychiatric readmission (or referral to community crisis services) in adult mental health inpatients. However no relevant literature was found. Finally, Lawes et al. (2018) rapid review of safe staffing structures agreed with the (Rutter et al. 2015) findings that there is limited evidence about optimum staff numbers/ratios and a general lack of research, especially outside of adult inpatient services. Question 1: “What is the current evidence on the impact of mental health nurses’ skill mix across mental health services and patient outcomes. The second area of interest is to investigate to what extent current deployment models support the provision of safe, efficient patient care across mental health services. A deployment model is defined as strategies for deploying mental health nurses within services, for example, covering staff shortfalls by deploying nurses temporarily to unfamiliar wards at short notice (Oliveira et al. 2023). Especially, as continuity of nursing care with staff that patients are familiar with has been identified as an important characteristic when planning services (NHS Improvement 2018). Question 2: "What is the current evidence on the impact of current mental health nurse deployment models to support the provision of safe, efficient patient care across mental health services.