Collateral circulation is an important aspect of vascular insufficiency to any organ of the body, including the kidney. Some renal collateral pathways visualized by angiography have been illustrated in previous publications (1–4). Aortography in these cases, performed by various technics, usually showed occlusion of the main renal artery and numerous large collateral vessels converging into the kidney. The ureteral vessels represent a major collateral route, and these tortuous collateral vessels may indent the wall of the ureter to produce irregularities which can be seen on the pyelogram in some instances (2, 3). This report reviews the routes of renal collateral pathways and illustrates that technics directed toward their angiographic demonstration may represent the essential mode of establishing the presence of renal ischemia and/or may define more clearly the anatomic details of the vasculature involved. Anatomic Considerations Usually, each kidney is supplied by a single artery, although supernumerary renal arteries are present in about 30 per cent of the cases (5). Regardless of the number of renal vessels or their variational patterns, however, it is a current anatomic concept that after the vessels perforate the renal sinus they are “;end-arteries” with no collateral connections to neighboring segments. Occlusion of a renal artery will usually result in infarction of the renal segments which it supplies. Nevertheless, in some circumstances renal collateral circulation is able to maintain function and viability of the involved renal parenchyma. We have chosen to divide the possible routes of renal collateral circulation into two major groups: the extrarenal and the intrarenal routes (Fig. 1). The extrarenal routes are the arterial connections with the renal arteries prior to their perforation of the renal sinus, and the intrarenal collateral routes are connections with the renal vessels after they have entered the renal parenchyma as interlobar arteries. Extrarenal Collateral Routes The arterial connections of the extrarenal branches of the renal artery represent the proved effective pathways for renal collateral circulation. Detailed descriptions of the extrarenal vessels and their variations have been reported by both anatomic and angiographic studies (5, 6). The usual extrarenal branches are the inferior adrenal, the renal pelvic, the ureteral, and the capsular arteries (Fig. 2, A). Other additional extrarenal branches are found commonly as anatomic variations, such as inferior phrenic, superior and middle adrenal, gonadal, etc. (Fig. 2, B). There are numerous communications between the extrarenal branches of the renal artery and the other arteries arising separately from the aorta; some of the important vessels are the intercostal artery, the subcostal artery, the inferior phrenic artery, the adrenal arteries, the lumbar arteries, and others.