Using U.S. firm level panel data we simultaneously assess the contributions to productivity of three potential sources of research and development spillovers: geographic, technological, and product market (“horizontal”). To do so, we construct new measures of geographic proximity based on the distribution of a firm's inventor locations as well as its headquarters. We find that geographic location is important for productivity, as are technology (but not product) spillovers, and that both intra and inter–regional (counties) spillovers matter. The geographic location of a firm's researchers is more important than its headquarters. These benefits may be the reason why local policy makers compete so hard for the location of local R&D labs and high tech workers.