We measure the size of knowledge spillovers of R&D activities done by venture capital financed start-ups and compare them quantitatively to spillovers of conventional corporate R&D. We suggest a novel measure to identify the appropriate spillover pool based on backward citations which reflect learning between firms. Using panel data of U.S. firms we show that venture capital financed R&D generates significant spillovers on the patent production of other firms. Counterfactual estimates suggest that the spillovers generated by VC-financed firms are more than 50% larger than those generated by established companies. We address potential endogeneity concerns with an instrumental variable strategy using changes in federal and state tax incentives as instrumental variable for R&D and past fund raising as instrument for venture capital investment.