Marx's Theories of Surplus Value is the fourth volume of his monumental Das Kapital (Capital). Divided into three parts, this lengthy work reviews classic economic analyses of labour and value (Smith, Ricardo, Malthus, and others), focusing on the concept of "surplus value" - the difference between the full value of a worker's labour and the wages received for this labour. This is a key concept for Marx since in his view the capitalist maintains power through controlling surplus value.