Because agglomeration economies may create competitive advantage and each location has a unique array of agglomeration economies, where should firms locate? We combine fundamental economic and strategy concepts to: (1) determine when firms must locate proximately to access factor pools; (2) show that factor pools controlled by fewer firms are less useful to new entrants; and (3) demonstrate that certain firms risk aiding competitors when contributing to efficient factor pools. We find support for our predictions with a test on new U.S. manufacturing entrants from 1985 to 1994, using an empirical specification that separates agglomeration levels from agglomeration economies.