The prevalence of intracranial aneurysms is 2.3% (95% CI, 1.7-3.1%); most of these aneurysms are small and located in the anterior circulation. Risk factors are age, female gender, smoking, hypertension, excessive use of alcohol, having one or more affected relatives with SAH and autosomal dominant polycystic kidney disease. Most studies on risk of rupture have methodological weaknesses; an important flaw is that observed risks are recalculated to yearly risks of rupture, assuming a constant risk of growth and rupture of aneurysms. In reality, it is much more likely that aneurysms have long periods of low risk and short periods of high risk of growth and rupture. The overall risk of rupture found in follow-up studies is around 1% per year. Size is the most important risk factor for rupture, with smaller risks for smaller aneurysms. Other risk factors are the site of the aneurysm (higher risk for posterior circulation aneurysms), age, female gender, population (higher risks in Finland and Japan) and, probably also, smoking. There are no good comparisons between clipping and coiling of unruptured aneurysms. Both treatment modalities have a risk of around 6% of complications leading to death or dependence of help for activities of daily living for aneurysms smaller than 10mm. These risks increase with larger size of aneurysms. For clipping, the risk seems to increase with age, for coiling this is less apparent. The efficacy of coiling on the long term is unsettled. In deciding whether or not to treat an aneurysm, life expectancy is a pivotal factor; other important factors are the size and the site of the aneurysm. If the aneurysm is left untreated, follow-up imaging may be considered to detect growth of aneurysms, but the frequency and effectiveness of repeated imaging are unknown.