Picture a bank offering you a one‑year deposit at 3% today, a two‑year deposit at 3.5%, and a ten‑year bond yielding 4%. These numbers form a curve — the term structure of interest rates — and they determine the price of almost every bond. In this chapter we build the tools to describe, model, and predict that whole curve in a consistent, no‑arbitrage way, so we can price bonds, swaps, and options on interest rates.