Christaller’s central place theory has limited application to contemporary urban systems for a variety of reasons. Perhaps the most obvious reason is that it envisions an economy where most people are scattered across the landscape in a dispersed pattern and cities exist primarily to serve the needs of those people. The current reality – certainly in the developed world but even in an increasing share of the developing world – is that most people live in cities. Thus, most goods and services provided in cities must be intended for an urban market; either they are sold to people within the city in which they are provided or they are sold to people in other cities. This means that it is necessary to think about two things that central place theory does not address: first, flows of goods and services among economic agents within the city and, second, flows of goods and services between cities. In this chapter, we focus on the latter.