An inflammatory condition of the APPENDIX which is a common surgical emergency, affecting mainly adolescents and young adults. It is usually due to a combination of obstruction and infection of the appendix, and has a variable clinical course ranging from episodes of mild self-limiting abdominal pain to life-threatening illness. Abdominal pain, beginning in the centre of the abdomen but which later shifts position to the right iliac fossa, is the classic symptom. The patient usually has accompanying fever and sometimes nausea, vomiting, loss of appetite, diarrhoea, or even constipation. The precise symptoms vary with the exact location of the appendix within the abdomen. In some individuals the appendix may ‘grumble’ with repeated mild attacks which resolve spontaneously. In an acute attack, the inflammatory process begins first in the wall of the appendix but, if the disease progresses, the appendix can become secondarily infected and pus may form within it. The blood supply may become compromised and the wall becomes gangrenous. Eventually the appendix may rupture, giving rise to a localised abscess in the abdomen or, more rarely, free pus within the abdomen, which causes generalised PERITONITIS. Rupture of the appendix is a serious complication and the patient may be severely unwell. Surgeons recognise that in order to make sure patients with appendicitis do not progress to peritonitis, a certain percentage of normal appendixes are removed when clinical signs are suspicious but not diagnostic of disease.