Critically ill patients have conditions that are considered life-threatening and require comprehensive care and constant monitoring; nutritional support plays a key role in the recovery of these patients and is an area of veterinary medicine that is very easy for the registered veterinary nurse (RVN) to have an active role in. Critically ill patients are at increased risk of malnutrition; acute and chronic illness, trauma and inflammation induced stress-related catabolism, and drug-induced adverse effects may reduce appetite or increase nausea and vomiting. Challenges exist in the provision of support, especially in the anorexic patient. This article focuses on how severe physiological stress affects animals that are critically ill and how this might lead to malnutrition, how to accurately calculate energy requirements, and discusses the importance of selecting the most appropriate diet to improve patient outcomes.