In the U.S., metropolitan areas contain the largest concentrations of people and jobs. Despite some drawbacks, these so-called agglomeration economies also have benefits, such as the cost savings that result from being close to suppliers and workers. Spatial concentration is even more pronounced among establishments that do basic research and development (R&D). In this article, we show that geographic concentration of R&D extends beyond locations such as Silicon Valley. In fact, many types of R&D establishments are highly concentrated geographically.