This allows us to quantify the normally unobservable forces that determine firm boundaries; that is, which transactions are mediated by ownership control, as opposed to contracts or markets. We find firm boundaries to be an economically significant barrier to trade: Having an additional vertically integrated establishment in a given destination ZIP code has the same effect on shipment volumes as a 40 percent reduction in distance. These effects are larger for high value-to-weight products, for faraway destinations, for differentiated products, and for IT-intensive industries.