Among patients with renal failure, those with ARF and critical illness represent by far the largest group undergoing artificial nutrition. ARF, especially in the ICU, seldom occurs as isolated organ failure but rather is a component of a much more complex metabolic environment, in the setting of the multiple organ failure. Nutritional programs for ARF patients must consider not only the metabolic derangements peculiar to renal failure and with the underlying disease process/associated complications, but also the relevant derangements in nutrient balance due to renal replacement therapies, especially when highly efficient renal replacement therapies (RRT) are used, such as continuous veno-venous hemofiltration (CVVH), or prolonged intermittent modalities such as sustained low-efficiency dialysis (SLED). Finally it is to be taken into account that nutrient requirements can change considerably during the course of illness itself (see also guidelines on PN in intensive care). From a metabolic point of view, patients with CKD or on chronic HD who develop a superimposed acute illness should be considered to be similar to patients with ARF. The same principles in respect of PN should therefore be applied.