A rare but potentially dangerous form of abdominal wall hernia that occurs just to the lateral aspect of the rectus muscle at the level of or just below the umbilicus. This hernia is a partial thickness defect of the abdominal wall. The two deepest layers (internal oblique and transversus abdominus) are separated but the most superficial (external oblique) remains intact. As a result, Spigelian hernias are difficult to appreciate on physical exam and require a high degree of suspicion. The diagnosis can almost always be made with a CT scan of the abdominal wall.