Why do two bonds from different companies, both promising to pay back the same amount at the same time, trade at very different prices? The short answer is credit risk. But the deeper puzzle is that the extra yield investors demand — the credit spread — is always changing, sometimes in ways that seem unrelated to how healthy the companies actually are. This chapter gives you a practical set of tools to understand those moves, measure how sensitive a bond is to the market, and spot when a spread looks too wide or too narrow compared to its fair value — the art of relative value analysis.