Chapter 3 discusses sovereign debt theory and practice. It goes through the history of sovereign debt and how the current theories of borrowing and lending developed in the 1980s. I argue that countries want to be part of global society, and that means they sometimes repay unsustainable debt. The chapter dives into why countries might default, when they might default, how often countries have defaulted, and what the economic and political costs are. I then describe what happens when countries need to restructure their sovereign debt, both in theory and with a practical guide for the process. Finally, in another technical section, I describe a sovereign debt model. The model explains when countries should have no willingness to repay their debt. It allows me to characterise a set of stylised macroeconomic facts that usually accompany sovereign debt defaults. The default set that comes out of the model states when countries should default. These facts and default set (not part of the technical section) are used in Chapters 6, 8, and 10. Chapter 3 is the last overview chapter; the rest are case studies.