The kidney and the liver play a central role in protein metabolism. Synthesis of albumin and other proteins occurs mainly in the liver, whereas protein breakdown and excretion are handled through an intricate interaction between these two organ systems. Thus, disease states of either the liver and/or the kidney invariably result in clinically relevant disturbances of protein metabolism. Conversely, metabolic processes regulated by these two organs are directly affected by dietary protein intake. Of particular importance in this respect is the maintenance of acid/base homeostasis. Finally, both the amount and composition of ingested proteins have a direct impact on renal function, especially in a state of diseased kidneys. Consequently, dietary protein intake is of paramount importance in patients with chronic nephropathy and renal insufficiency. Limitation of ingested protein, particularly from animal sources, is crucial in order to slow the progression of chronic kidney disease and impaired renal function. In contrast, patients with chronic renal failure undergoing renal replacement therapy by hemodialysis or peritoneal dialysis, have an increased protein demand. The syndrome of "protein-energy malnutrition" is a relevant factor for morbidity and mortality in this population and requires early detection and vigorous treatment. Protein intake in patients with cirrhosis of the liver should not be diminished as has been earlier suggested but rather increased to 1.0 - 1.2 g/kg body weight/day, in order to prevent protein malnutrition. Moderate restriction depending on protein tolerance (0.5 - 1.2 g/kg body weight/day), with the possible addition of branched chain amino acids (BCAA), has been recommended only in patients with advanced hepatic encephalopathy. Proteins of plant origin are theoretically superior to animal proteins.