This book undertakes a fundamental review of the existing international system of taxing business profit. It steps back from the current political debates on how to combat profit shifting and how taxing rights over the profits of the digitalized economy should be allocated. Instead, it starts from first principles to ask how we should evaluate a tax on business profit-and whether there is any good rationale for such a tax in the first place. It then goes on to evaluate the existing system and a number of alternatives that have been proposed. It argues that the existing system is fundamentally flawed, and that there is a need for radical reform. The key conclusion from the analysis is that there would be significant gains from a reform that moved the system towards taxing profit in the country in which a business made its sales to third parties. That conclusion informs two proposals that are put forward in detail and evaluated: the Residual Profit Allocation by Income (RPAI) and the Destination-based Cash Flow Tax (DBCFT).