We argue that when innovation is sequential (so that each successive invention builds in an essential way on its predecessors) and complementary (so that each potential innovator takes a different research line), patent protection is not as useful for encouraging innovation as in a static setting. Indeed, society and even inventors themselves may be better off without such protection. Furthermore, an inventor's prospective profit may actually be enhanced by competition and imitation. Our sequential model of innovation appears to explain evidence from a natural experiment in the software industry. Copyright (c) 2009, RAND.