The pandemic increase in obesity prevalence leads to a large increase in the number of obese patients admitted in the intensive care unit. The “obesity paradox” defines the protective effect of overweight and obesity (until 40 of body mass index) on intensive care unit mortality. In critically ill patients, obesity-related mortality is likely to be underestimated because of the use of non-suitable prognostic scores. During critical illness, the adipose tissue of obese patients stores lipids instead of utilizing them as energy source. It leads to the worsening of the muscle protein catabolism. The metabolic response to critical illness in obese patients may involve qualitative changes of the adipose tissue. Hyperglycemia is frequent and associated with patients’ poor prognosis making mandatory the glycemic control. Overnutrition must be avoided as it is deleterious. The main objective of nutritional support is to limit the malnutrition secondary to critical illness. In the critically ill obese patients, no method is validated for assessing nutritional status and protein–energy needs. The strategy of nutrition support is still largely debated, including the use of hypocaloric–hyperproteic feeding. The scarcity of scientific data makes urgent the biomedical research on the topic of the critically ill obese patients.