Water accounts for approximately half of an adult human's body weight. Two thirds of body water is intracellular, and the remaining one third is contained in the extracellular fluid compartment, which includes intravascular (plasma) and interstitial fluid. Small amounts of water are also contained in bone, dense connective tissue, digestive secretions, and cerebrospinal fluid. To maintain stability of the internal milieu, body fluids are processed by the kidney, guided by intricate physiologic control systems that regulate fluid volume and composition. This chapter reviews the principles of body fluid hemostasis and discusses disorders of water excess (hyponatremia), water deficiency (hypernatremia), water conservation (diabetes insipidus), saltwater excess (edematous states), and saltwater deficiency (volume depletion). Figures illustrate the relation between the plasma hydrogen ion concentration and the blood pH, proximal tubular bicarbonate reabsorption, renal secretion of hydrogen ions, renal glutamine metabolism and ammonia diffusion, metabolic alkalosis, and the electrocardiographic changes in and approach to causes of hypokalemia and hyperkalemia. Tables describe causes of metabolic acidosis with a high and a normal ion gap; causes of type 1 renal tubular acidosis; causes of type 4 renal tubular acidosis and aldosterone resistance; causes of metabolic alkalosis, respiratory acidosis, and respiratory alkalosis; evaluation of the renal defect in hyperkalemia; and treatment of hyperkalemia. This chapter contains 10 figures, 12 tables, and 53 references.