This volume discusses renal function and the mechanisms by which the kidney regulates the composition and volume of the extracellular fluid. It also highlights the role of the kidney in the development and progression of arterial hypertension. Most textbooks of renal physiology are based in mammalians physiology and mostly human physiology of the kidney, but the authors considered that this book should also include other species to include the broad spectrum of students and researchers in the life and biomedical sciences. In this sense, we included chapters such as comparative osmoregulation in non-mammalian vertebrates and we emphasize that in vertebrates like fish, reptiles, amphibians and birds, the kidneys and extrarenal organs are vital to maintain fluid homeostasis. The purpose of the book is to provide a concise frame of knowledge in a clear and direct language, of the renal function to medical and biological sciences students.