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 extra renal 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. In the context of normal renal function, we provide pathophysiological basis for chronic renal diseases and hypertension with the participation of renal vasoactive hormones. This book is used as textbook in several physiology courses for medical, nursing and biological sciences students at the Pontifical Catholic University of Chile, Finis Terrae University, Universidad San Sebastian as well as other universities.