In the economic literature, worker co-operatives are viewed as small, specialised and undercapitalised organisations that only thrive in unusual conditions and could not possibly constitute a serious alternative to conventional firms. This chapter challenges this conventional wisdom, drawing on a range of international economic studies comparing worker co-operatives and conventional firms. The evidence reviewed suggests that worker co-operatives match ‘mainstream’ firms for durability, and can be found in most industrial sectors. The chapter rejects the notion that worker co-operatives are systemically undercapitalised, and asserts that many are more productive than their mainstream equivalents, largely because of their capacity to motivate workers through their control of the enterprise. It suggests that worker co-operation is a viable alternative to private ownership for small and medium sized enterprises (SMEs), and that in some regions of Italy and Spain, the model is integrated into the economic mainstream.