Nations have been trading goods and capital for hundreds of years, but it is only very recently that a combination of technological evolution, statistical progress and theoretical work has allowed economists to analysis capital flows as a stand-alone topic. The balance of payments is at the heart of this inquiry. In it simplest form, the BOP is nothing but a crude double-entry accounting framework, but look closer and a much more complex picture emerges. The transactions recorded on the BOP are subject to both economic and political controversy. This essay takes these issues seriously by showing why the transition from closed to open economies is a profound shift, both in terms of macroeconomic analysis, but also in the context of international relations between nations playing out an iterated game of exchange over time. The first edition of this essay was first published in 2019. Other editions might follow and will be available here.