This chapter discusses mechanism of urine formation, which involves three basic processes such as filtration, reabsorption and secretion. The first step in the formation of urine is filtration through the glomerular capillaries. The filtration surface is a three layered structure which include endothelium, basement membrane, and epithelium. Hydrogen ion is the only substance which cannot be adequately got rid of by filtration. Potassium ions are secreted passively along an electrical gradient. The clearance technique can be applied to any substance handled by the kidneys. Clearance data can tell us quite a lot about the mechanism of excretion of substances. Since the threshold concentration is considerably above the normal glucose concentration, normally no glucose is excreted in the urine. The medullary region of the collecting duct is permeable to urea. Renal tubules handle the tubular load typically by massive reabsorption. The glomerular filtrate is passed on to the proximal tubule.