Despite advances in the treatment of abdominal aortic aneurysms (AAAs), the pathogenesis of aneurysmal disease remains obscure. Aneurysm screening programmes have successfully identified a proportion of patients with early or small aortic aneurysms,~-3 who do not fulfil the current criteria for elective aneurysm repair on the basis of aortic diameter. 4 At present these patients are managed conservatively, with regular ultrasonographic follow-up, but with increased knowledge of aneurysm pathophysiology, it is possible that aneurysm growth may be retarded with medical or gene therapy. 5 This review presents the current theories of aneurysm pathogenesis, with emphasis on the cellular and biochemical events within this disease process.