When a foreign person earns income from sources inside the United States, the U.S. tax system faces a practical problem: how do you collect tax from someone who might have no office, no assets, and no presence in the country? The answer is withholding at source — making the person who pays the income responsible for taking the tax out before the money ever leaves the country. This chapter explores exactly which types of income are caught by this withholding net, the important exceptions that let certain interest payments flow freely, and the special regime that grabs gains from U.S. real estate even when other capital gains escape untouched.