Gallstones are calculi formed in the gallbladder or bile duct. There are three main types of stone - pure cholesterol, pigment and the commonest, mixed stones. There is usually a small core of organic material, often containing bacteria. The main part of the stone is made up of concentric layers, suggesting that it is formed by a series of discrete precipitation events. Only 20% are radio-opaque.

Up to 20% of people in the West have gallstones, although many remain asymptomatic throughout life. This compares to a 4% prevalence in the developing world.

Women are affected twice as often as men; pregnancy, obesity and diabetes are important predisposing factors. The typical patient is said to be the "fair, fat, fertile, female of forty", but many gallstone patients do not fit this description.