The relation between the intensity of competition and R&D investmenthas received a lot of attention, both in the theoretical and in the empirical literature. Nevertheless, no consensus on the sign of the effect of competition on innovation has emerged. This survey of the literature identifies sources of confusion in the theoretical debate. My discussion is mainly based on a unified model that simplifies the comparison of different results. This model is also applied to show which factors work in favor of a positive relation between competition and innovation.