The urinary system is also known as renal system, which consists kidneys, ureters, bladder, and the urethra. Kidneys are paired, bean-shaped organs in the retroperitoneal space. Each adult kidney weighs about 150 g. They are covered by a thin capsule. The nephron is the functional and structural unit of the kidney. Nephron consists of glomerulus, tubule, and the collecting system. Congenital and developmental anomalies, cystic diseases of the kidney, glomerular diseases, tubular and interstitial diseases, vascular diseases, urinary tract obstruction (obstructive uropathy), and neoplasms of the kidney are the classifications of renal diseases. Pyelonephritis and urinary tract infection, tuberculosis of the urinary tract, diabetic nephropathy, renal vascular diseases, urinary tract obstruction (obstructive uropathy), chronic kidney disease, renal replacement therapies, malignant tumors of the kidney, and urothelial tumors are also discussed in this chapter.