The peritoneum exists as a closed sac within a second closed sac, the fascial layer derived from and continuous with transversalis fascia. Extraperitoneal abscess may occur between the peritoneum and the deep fascia anteriorly, posteriorly, or in the pelvis. Extrafascial abscess is a form of extraperitoneal abscess that develops outside the transversalis layer and may occur anteriorly a well as posteriorly. A classification of extraperitoneal infection is proposed, which is both clinically applicable yet reflects the anatomic relationship of the peritoneum and transversalis fascia.