Imagine you earn money from different countries: interest from a German bank, salary for work you do in Canada, profit from selling shoes in Mexico. Every tax system has to decide: is this income from inside the country or from outside? The answer determines the income’s source—whether it’s U.S.-source or foreign-source. That decision drives who owes tax, how much, and whether you can claim a credit for taxes paid to another country. This chapter walks through the rules that sort income as U.S. or foreign, a foundation for almost every international tax calculation.