Kidneys are remarkable organs, and they play a pivotal role in whole-body homeostasis. In short, the kidneys regulate the extracellular environment, speci”cally extracellular water and electrolytes. In addition, they eliminate nitrogenous products from protein metabolism (urea) and creatinine, which is released from skeletal muscle. Mostly everyone, laypersons and professionals alike, recognizes the importance of the elimination function of the kidneys and the fact that the failure of the kidneys to eliminate urea (uremia or azotemia) does indeed cause profound pathologic disturbances. However, the ability of the kidneys to maintain extracellular water and electrolytes within an acceptable range has a profound in¬uence on all the organs of the body because all cells reside in the extracellular environment. Retaining extracellular volume, for example, can result in hypertension, and disturbance in kidney function is one known cause of this disease (Dibona, 2002; Ganong, 2006; Kotchen, 2008). In fact, the use of diuretic therapy to treat hypertension illustrates the connection between the kidneys and blood-pressure regulation (Hoffman, 2006). Failure of the kidneys to eliminate or retain hydrogen and/or bicarbonate can result in acid-base disturbances, which of course affects the function of other organs of the body (Rose and Post, 2001). Extracellular potassium must be maintained within a narrow range; otherwise, fatal arrhythmias can result. In this chapter, we review some of the basic physiological processes of the kidneys, thus providing a foundation for understanding how alterations in these processes result in pathologic changes that occur in patients with renal disease. Understanding this underlying physiology also aids in making therapeutic and nutritional choices in treating patients with kidney disease.