There are two types of stroke and they have different causes.
An ischaemic stroke is the most common type of stroke. It is usually caused by a blood clot in an artery that supplies blood to your brain.
A clot may form in an artery, in the brain itself, or a clot that has formed in a larger artery in your chest or neck that may break away and be carried by the bloodstream to a smaller artery in your brain where it becomes lodged.
Clots tend to form in arteries that have become narrowed by the slow build up of fatty material called 'plaque' or 'atheroma'. This gradual clogging process is known as 'atherosclerosis', and is the same process that causes coronary heart disease.
Haemorrhagic strokes happen when an artery in your brain bursts. They lead to bleeding in your brain and squashing of the tissue around the broken artery.
This type of stroke is usually caused by high blood pressure and/or diseases involving the blood vessels in your brain.