Parenteral nutrition (PN) is a technique of artificial nutrition support, which consists of the intravenous administration of macronutrients, micronutrients, and water. PN has become integrated into intensive care unit (ICU) patient management with the aim of preventing energy deficits and preserving lean body mass. The addition of PN to enteral nutrition is known as supplemental PN. Parenteral feeding should be considered whenever enteral nutritional support is contraindicated, or when enteral nutrition alone is unable to meet energy and nutrient requirements. International guidelines differ considerably regarding the indications for PN. Thus, the ESPEN guidelines recommend initiating PN in critically-ill patients who do not meet caloric goals within 2–3 days of commencing EN, while the Canadian guidelines recommend PN only after extensive attempts to feed with EN have failed. The ASPEN guidelines advocate administering PN after 8 days of attempting EN unsuccessfully. Several studies have demonstrated that parenteral glutamine supplementation may improve outcome, and the ESPEN guidelines give a grade A recommendation to the use of glutamine in critically-ill patients who receive PN. Studies on IV omega-3 fatty acids have yielded promising results in animal models of acute respiratory distress syndrome and proved superior to solutions with omega -6 compositions. The discrepancy between animal models and clinical practice could be related to different time frames.