This paper explores the complex interrelationships between knowledge and decision making as nurses and doctors interacted with each other in a critical care unit, which comprised a combined general intensive care and cardiothoracic surgical unit. The critical ethnographic study upon which this paper is based, involved a research group of six nurses who worked in the unit. Nurses differentially valued their knowledge, depending on the situation, experience and level of medical input. They were also involved in decision making based on their differential visibility in the process. Nurses' specialised knowledge of the critical care unit played a major role in influencing how they interacted during decision making.