The materials ground by millers are often foodstuffs and particularly grain. The physical grinding of the food allows for the easier digestion of its nutrients and saves wear on the teeth. Non-food substances needed in a fine, powdered form, such as building materials, may be processed by a miller.
The most basic tool for a miller was the quern-stone—simply a large, fixed stone as a base and another movable stone operated by hand, similar to a mortar and pestle. As technology and millstones (the bedstone and rynd) improved, more elaborate machines such as watermills and windmills were developed to do the grinding work. These mills harnessed available energy sources including animal, water, wind, and electrical power. Mills are some of the oldest factories in human history, so factories making other items are sometimes known as mills, for example, cotton mills and steel mills. These factory workers are also called millers.