The article reassesses the contribution of Sir Arthur Lewis's 1954 article entitled "Economic Development with Unlimited Supplies of Labour" to our understanding of economic development, and to the establishment of development economics as an academic discipline. It argues that Lewis's key insight into the structural dualism characterising developing countries mapped out a new and distinctive field for development economics and policy. The article has had a profound impact over the last fifty years and continues to yield valuable lessons for understanding the nature of economic transformation.