Hernias and hydroceles are common conditions of infancy and childhood, and inguinal hernia repair is one of the most frequently performed pediatric surgical operations. A hernia is defined as the protrusion of a portion of an organ or tissue through the wall normally containing it. Three types of hernias occur in the groin, indirect inguinal hernias, direct inguinal hernias, and femoral hernias. As a result of improved neonatal intensive care, more and more premature babies are being delivered, and consequently the incidence of neonatal inguinal hernia and hydrocele is increasing. The most important aspect of inguinal hernia relates to the risk and management of incarceration and emphasis will be placed on this point during the course of this article. This article also covers the embryology, incidence, clinical presentation, and treatment of groin hernias and hydroceles, and also discusses abdominal wall hernias other than umbilical. For a description of this type of hernia see the article by O'Donnell elsewhere in this issue.