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We review the role of R&D in endogenous growth theory, and describe extant empirical research---macro and micro---bearing on R&D as an engine of growth. Taking R&D to be key while recognizing the significance of economic incentives emphasizes understanding, in general, knowledge as an economic object and, in particular, the economics of intellectual property rights. This paper argues property rights matter, but in subtle, counterintuitive ways not yet fully investigated in research on endogenous growth.