Although the anti-cancer effects of aspirin were first identified in pre-clinical models four decades ago, a clear role for the drug in either the prevention or treatment of cancer has not been established. Concerns about toxicity, particularly major haemorrhage, and a lack of randomised evidence demonstrating efficacy have limited its use in primary prevention; there was also doubt that a simple aspirin could have a significant therapeutic effect against established malignancy. Three new pieces of evidence: a series of meta-analyses focusing on cancer outcomes from randomised-controlled trials designed to assess the vascular benefits of daily aspirin; the first positive results from a randomised-controlled trial designed to demonstrate that aspirin can prevent cancer in those with a hereditary predisposition; and observational data showing that aspirin use after a cancer diagnosis improves both cancer mortality and overall survival; have led to a re-evaluation of aspirin as a potential anti-cancer agent both for the prevention and treatment of cancer.