An aneurysm is a localised dilation of the wall of a blood vessel, usually caused by atherosclerosis and hypertension, or less frequently, by trauma, infection or a congenital weakness in the vessel wall. Aneurysms most commonly occur in arteries at the base of the brain and in the aorta. As the size of the aneurysm increases, there is an increased risk of rupture, which can result in severe haemorrhage and other complications including sudden death. This book describes different types of aneurysms, their causes, diagnosis and how they can be treated.