Health care professionals use policies and protocols in varying ways to guide their clinical activities and to promote quality patient care. The critical ethnographic case study upon which this paper is based, involved a research group comprising six registered nurses who worked in a critical care setting. Research methods included professional journalling, participant observation, and focus group and individual interviews. This paper examines the power relations at play between doctors and nurses, and among nurses, and the ways in which nurses used policies and protocols as a means of mediating communication. While policies and protocols provided nurses with legitimacy of their knowledge in the clinical arena, doctors tended to rely on their past experience and background to inform their knowledge and activities. For nurses to believe that they provided valued and collaborative input in patient decisions, they actively sought out written evidence through policies and protocols to confirm and support their knowledge. Policies and protocols of critical care activities provided nurses with expected standards of care, which they used to legitimize their knowledge and to communicate with doctors about 'undesirable' medical decisions. The doctors valued their professional authority and autonomy over policies and protocols, while nurses used these written guidelines to assert power and demonstrate resistance. Policies and protocols do not exist in isolation; they occur within a complex network of power relations that create tensions in clinical practice. In challenging these tensions, it is important that nurses and doctors establish a fine balance between using policies and protocols to provide directions for practice, and to allow sufficient latitude and flexibility in addressing the complexities of patient care.