Int. J. Adv. Res. 7(12), 213-227 214 Familiarity with the CT and MR imaging features of various retroperitoneal masses will facilitate accurate diagnosis as well as staging of these lesions.This pictorial essay reviews the imaging features of few of the histopathologically confirmed common and uncommon primary retroperitoneal tumors presented to our institute from 2016 to 2019. Anatomy of retroperitoneumThe retroperitoneum is a compartmentalized space extending between transversalis fascia posteriorly and posterior parietal peritoneal layer anteriorly [ Fig. 1]. It is divided into following three potential spaces: a) Anterior pararenal space, b) posterior pararenal space c) perirenal space and d) great vessel space. Anterior pararenal space is bounded anteriorly by posterior parietal peritoneum, posteriorly by anterior renal fascia / Gerota's fascia and laterally by lateroconal fascia which is a continuation of posterior renal fascia / Zuckerkandl fascia Anterior pararenal space is subdivided into (i) pancreaticoduodenal space comprising pancreas and second, third and fourth portions of the duodenum, and (ii) pericolonic space comprising ascending and descending colon, lymph nodes and fat. Anterior pararenal space is continuous across the midline. Posterior pararenal space extends between Zuckerkandl fascia anteriorly and transversalis fascia posteriorly and contains only small amounts of fat. The perirenal space is situated between anterior and posterior renal fasciae and contains adrenal gland, kidney and renal pelvis, proximal ureter, renal hilar vessels, lymph nodes, and fat. The perirenal space contains bridging septa and a network of lymphatic vessels that allow the spread of disease to or from adjacent spaces. There are three more potential expansile planes composed of apposed layers of embryonic mesentery formed from laminations of renal and lateroconal fasciae: (i) retromesenteric, (ii) retrorenal and (iii) lateroconal planes, formed by splitting of anterior renal fascia, posterior renal fascia and lateroconal fascia respectively. The great vessel space is a fat containing space located anterior to psoas muscles and vertebral bodies that surrounds the aorta and inferior vena cava. A fifth space, the infrarenal retroperitoneal space is formed by the fusion of anterior and posterior pararenal spaces, below the level of the kidneys and is communicating with the prevesical and extraperitoneal compartments of the pelvis. 4,5,6,7