A hernia is the protrusion of an organ or piece of tissue from its normally contained space. In the abdomen, a hernia usually involves a piece of bowel, or omentum, which is a fatty apron that hangs down from the midportion of the colon (large intestine), protruding through a weak area in the abdominal wall. The abdominal wall is made up of layers of different muscles and tissues. Weak spots may develop in these layers to allow contents the abdominal cavity to protrude and herniate. The most common hernias are in the groin (inguinal hernias) and in the diaphragm (hiatal hernias). Hernias may be present at birth (congenital), or they may develop at any time thereafter (acquired).