Perforated appendicitis with an abscess formation is known to be the most common cause of tender mass in right lower abdomen. The differential diagnosis of a tender mass in right lower abdomen, however, is broad and includes various intraabdominal and intrapelvic lesions in origin. Preoperative accurate diagnosis is essential in order toavoid unnecessary surgery or complications resulting from delayed surgery. The purpose of this pictorial essay is to illustrate examples of various causes and methods to differentiate them with ultrasonography and CT.