Disorders of water balance lead either to dehydration or overhydration. Because there is an intimate relationship between water and sodium concentration (water generally following salt), one can distinguish hypotonic, isotonic and hypertonic dehydration and the same for overhydration. The vast majority of water balance disorders are acquired. In this article, the focus is on the inherited disorders both of water (nephrogenic diabetes insipidus) and acid-base balance. Both acidosis and alkalosis can arise from primary tubular ion transport abnormalities. The alkaloses are usually secondary to salt handling problems, whereas the renal tubular acidoses are often a consequence of primary abnormalities of acid-base transporters.