In spite of its rare incidence, the inherited renal tubular disorder nephrogenic diabetes insipidus (NDI) has in the past 10 years attracted the attention of a varied group of medical doctors and basic scientists. With the identification of the two genes involved in NDI, namely the vasopressin type-2 receptor (V2R) and the aquaporin-2 water channel (AQP2) genes, the identification of a large number of different mutations in these genes, and the subsequent functional characterization of mutant V2Rs and AQP2s, our insight into the renal cellular mechanisms involved in diuresis and antidiuresis has increased considerably. We are now entering an exciting new period in the development of new therapeutic strategies for disorders of water balance.