The retroperitoneum (adj. retroperitoneal) 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.
Organs that were once suspended within the abdominal cavity by mesentery but migrated posterior to the peritoneum during the course of embryogenesis to become retroperitoneal are considered to be secondarily retroperitoneal organs.
Retroperitoneal structures
Structures that lie behind the peritoneum are termed "retroperitoneal". These include: