We focus on two themes, among those in Mansfield's work, particularly relevant to understanding the role of large corporations in the U.S. innovation system: (1) the development of science-based inventions into market-ready innovations, and (2) the imitation by one firm of another's technology. Both of these phenomena, we propose, depend critically on the extent of technological and organizational complexity characteristic of current products and potential innovations. Reporting on recent survey research of our own, we argue that the origins and potentially the future of U.S. leadership in technology-based economic growth lie in the complementarity of large corporations and entrepreneurial start-ups, each exploring and exploiting the market potential of different types of science-based innovations.