This chapter discusses factors that cause interest rates to change over time. It examines the forces that move interest rates using a supply and demand framework for bonds. The demand for bonds depends on wealth, expected returns, risk, and liquidity. The supply depends on expected profitability, expected inflation, and government activities. Changes in these factors can shift the supply and demand curves for bonds and change the equilibrium interest rate. The chapter analyzes examples like the Fisher effect and business cycle expansions to demonstrate how interest rates are determined.