This paper reviews economic principles for optimality of the taxation of international profit, from both a global and national perspective. It argues that for traditional systems based on the residence of the investor or the source of the income, nothing less than full harmonization across countries can achieve global optimality. The conditions for national optimality are more difficult to identify, but are most likely to imply source-based taxation. However, source-based taxation requires an allocation of the profits of multinational companies to individual jurisdictions; this is not only very difficult in practice, but in some cases is without any conceptual foundation. The taxation of interest income on a residence basis is also hard to justify if the aim of the tax system is to tax only the income arising from economic activity in a given country.