Nutritional assessment of companion animals is considered to be the fifth vital sign and nutritional support is an important aspect of care for animals in the intensive care unit (ICU). Malnutrition can develop quickly, even in adequately nourished but critically ill animals, and typically results from the type, severity and metabolic demands of the disease affecting the animal, demands from healing, and rapid fluid and electrolyte shifts, which could potentially result in deficiencies in micronutrients such as potassium and magnesium, and loss of specific vitamins and minerals. Adequate nutritional support can preserve lean body mass, facilitate wound healing, reverse the maladaptive metabolic response to injury, maintain organ function and reduce morbidity in critically ill animals. However, nutritional support, whether administered enterally or parenterally, can result in mechanical, infectious and metabolic complications. Therefore, the aims of nutritional support in the ICU include preserving lean body mass, correcting or preventing macro- or micronutrient deficiencies and preventing complications associated with the provision of nutritional support. This article discusses the whole procedure of nutritional intervention, from deciding which route to use to the selection of diets that can be fed and the support required for specific diseases and conditions.