A factory, manufacturing plant or production plant is an industrial facility, often a complex consisting of several buildings filled with machinery, where workers manufacture items or operate machines which process each item into another. They are a critical part of modern economic production, with the majority of the world's goods being created or processed within factories.

Factories arose with the introduction of machinery during the Industrial Revolution, when the capital and space requirements became too great for cottage industry or workshops. Early factories that contained small amounts of machinery, such as one or two spinning mules, and fewer than a dozen workers have been called "glorified workshops".

Most modern factories have large warehouses or warehouse-like facilities that contain heavy equipment used for assembly line production. Large factories tend to be located with access to multiple modes of transportation, some having rail, highway and water loading and unloading facilities. In some countries like Australia, it is common to call a factory building a "Shed".

Factories may either make discrete products or some type of continuously produced material, such as chemicals, pulp and paper, or refined oil products. Factories manufacturing chemicals are often called plants and may have most of their equipment – tanks, pressure vessels, chemical reactors, pumps and piping – outdoors and operated from control rooms. Oil refineries have most of their equipment outdoors.

Discrete products may be final goods, or parts and sub-assemblies which are made into final products elsewhere. Factories may be supplied parts from elsewhere or make them from raw materials. Continuous production industries typically use heat or electricity to transform streams of raw materials into finished products.

The term mill originally referred to the milling of grain, which usually used natural resources such as water or wind power until those were displaced by steam power in the 19th century. Because many processes like spinning and weaving, iron rolling, and paper manufacturing were originally powered by water, the term survives as in steel mill, paper mill, etc.

Max Weber considered production during ancient and medieval times as never warranting classification as factories, with methods of production and the contemporary economic situation incomparable to modern or even pre-modern developments of industry. In ancient times, the earliest production limited to the household, developed into a separate endeavor independent to the place of inhabitation with production at that time only beginning to be characteristic of industry, termed as "unfree shop industry", a situation caused especially under the reign of the Egyptian pharaoh, with slave employment and no differentiation of skills within the slave group comparable to modern definitions as division of labour.

According to translations of Demosthenes and Herodotus, Naucratis was a, or the only, factory in the entirety of ancient Egypt. A source of 1983 (Hopkins), states the largest factory production in ancient times was of 120 slaves within fourth century BC Athens. An article within the New York Times article dated 13 October 2011 states: