Strike action, often simply calle a strike, is a work stoppage caused by the mass refusal by employees to perform work. A strike usually takes place in response to employee grievances. Strikes became important during the industrial revolution, when mass labour became important in factories and mines. In most countries, they were quickly made illegal, as factory owners had far more political power than workers. Most western countries partially legalized striking in the late 19th or early 20th centuries.