How do R&D incentives affect individual firms and, in turn, shape aggregate growth? We develop a novel empirical framework, grounded in endogenous growth theory, allowing us to measure firms’ responsiveness to R&D incentives and to aggregate such responses. After validating the predictions of our framework using three different micro-datasets, we apply it to Compustat data. We find that (i) ignoring firm heterogeneity severely under-states the aggregate effectiveness of R&D incentives, (ii) per dollar spent on R&D incentives, young (rather than small) firms raise aggregate growth the most and (iii) our results are robust to knowledge spillovers, dynamics and borrowing constraints.