The retroperitoneal space (retroperitoneum) is the anatomical space in the abdominal cavity behind (retro) the peritoneum. It has no specific delineating anatomical structures. Organs are retroperitoneal if they only have peritoneum on their anterior side. Structures which are not suspended by mesentary in the abdominal cavity and which lie between the parietal peritoneum and abdominal wall are classified as retroperitoneal.