"We introduce a tractable model of endogenous growth in which the returns to innovation are determined by the technology adoption decisions of the users of new technologies. Technology adoption involves an implementation investment that determines the initial productivity of a new technology. After implementation, learning increases the productivity of a technology to its full potential. In this framework, implementation enhances growth, while growth increases obsolescence and reduces implementation. In a calibrated version of our model, the optimal policy involves a subsidy to capital and to implementation and a R&D tax. This policy would lead to a welfare improvement of 7.6 percent. Out of steady-state analysis yields that the transitional dynamics of the detrended variables after a shock to capital are very similar to the dynamics of the neoclassical growth model, but transitory shocks have permanent effects on the level of productivity"--National Bureau of Economic Research web site.