In this article the osmoregulatory, acid-base homeostasis, and excretory functions of the renal system of invertebrates and vertebrates are reviewed. The mammalian renal system is the most highly evolved in terms of the range of functions performed by the kidneys. Renal physiology in other animals can be very different, and a sound knowledge of these differences is important for understanding health and disease processes that involve the kidneys, as well as ion and water homeostasis. Many animals rely on multiple organs along with the kidneys to maintain osmotic, ionic, and pH balance. Some animals rely heavily on postrenal modification of urine to conserve water and salt balance; this can influence the interpretation of disease signs and treatment modalities.