The aim of this book is to offer a precise account of practical reasons by developing and defending a new proposal called reason structuralism. The main argumentative strategy is abductive: It is shown that reason structuralism has theoretical advantages and greater explanatory power over other options. In particular, when it comes to theoretical advantages, reason structuralism provides a clear, simple theory that is theoretically economical and offers ontological neutrality, and is thus compatible with multiple substantive ethical theories. When it comes to explanatory power, reason structuralism is able to give an elegant account of reason aggregation and double counting, and it makes it possible to give an account of phenomena that arguably have not been studied so far for lack of tools, such as reason subtraction and partial reasons.