• Most common surgical emergency in pediatric patients, the incidence in the United States is 1 per 1000 children. • The appendix represents a long true diverticulum of the colon, a reservoir for normal intestinal flora with the highest concentration of gut-associated lymphoid tissue (GALT). • Inflammation of the appendix can be due to obstructive processes such as fecaliths, lymphoid hyperplasia, or neoplasia, but this is not sufficient to describe all of the cases of appendicitis. • Clinical course typically presents with anorexia and vague periumbilical pain that migrates to the right lower quadrant (RLQ) at McBurney point (1–2 in. from the anterior superior iliac process in the direction of the umbilicus). Patients can have nausea and vomiting. • Periumbilical pain is transmitted from visceral nerve irritation; as appendicitis becomes more severe and irritates the peritoneum, the pain becomes more localized to the RLQ as it is transmitted by somatic nerve fibers. • On a physical exam, patients will generally have pain in the RLQ with palpation, flexion, or movement. Rebound tenderness is also a common finding.  – Rovsing sign—RLQ pain that is transmitted from palpation in the LLQ.  – Obturator sign—RLQ pain from flexion and internal rotation at the hip.  – Psoas sign—RLQ pain when patient had the right leg extended in left lateral decubitus (can be a sign of a retrocecal appendicitis).  – Dunphy sign—RLQ pain with dorsiflexion of the right foot. • Laboratory studies are usually ordered and show inflammation with an elevated white blood cell count with a “left shift” indicative of active infection and elevated C-reactive protein. • Differential: gastroenteritis, constipation, urinary tract infection, sexually transmitted infection, mesenteric adenitis, Crohn’s disease, Meckel’s diverticulitis, and ureteral stones.  – In patients with female sexual organs—menstrual pain, ovarian cyst, ovarian torsion, and pregnancy.