A single home loan is a pretty unexciting investment—it’s tiny, hard to sell quickly, and packed with borrower-specific risk. But bundle thousands of mortgages together, slice the cash flows into pieces that suit different investors, and suddenly you have a multi‑trillion‑dollar market that shapes the cost of borrowing for almost everyone. This chapter explains how that transformation works: from the agencies that guarantee most U.S. mortgage‑backed bonds, to the clever structuring that turns a pool of loans into dozens of distinct securities, each with its own risk‑reward personality.