Appendicitis is defined as inflammation of the vermiform appendix. It is the most common abdominal surgical emergency and occurs at an annual rate of approximately one in 10,000 in the United States. The lifetime risk of appendicitis is about 9% for males and 7% for females; approximately 80% of cases occur before 45 years of age. Appendicitis rarely occurs in infants; it increases in frequency between 2 and 4 years of age and reaches a peak between the ages of 10 and 19 years. However, clinicians must maintain a high index of suspicion in patients of all age groups. This review covers the pathophysiology, stabilization and assessment, and diagnosis and treatment of complicated and uncomplicated appendicitis. The disposition and outcomes are also reviewed. Figures show an image of appendicitis on a bedside sonogram, and a computed tomographic image of appendicitis. Tables list likelihood ratios of signs and symptoms of appendicitis, the sonographic appearance of appendicitis, the Alvarado scoring system, and the differential diagnosis of appendicitis. This review contains 2 figures, 7 tables, and 36 references. Key words: appendicitis, obstructed appendiceal lumen, rebound abdomen, right lower quadrant pain, ruptured appendix