This work was undertaken to classify and study the blood vessels of the kidney, and to find out what percentage of kidneys have accessory blood vessels. The normal state as pictured in most books, is a pair of kidneys each with its one renal artery passing from the Aorta to the pelvis. Is this the state of affairs in the majority of cases, or is it the state in the minority? What reserve of blood supply has the kidney; what is the embryology of the vessels; what patt do the vessels play in high blood pressure; what part do they play in the production of hydronephrosis and infarcts; are the vessels prone to calcification; what are the relations of the renal vessels to those of the suprarenals from the point of view of the hormonic and physiological control of blood pressure? These are some of the many important questions to be answered from a study of the renal arteries. Some of my observations from the study of several hundred kidney systems are set down below.