This chapter analyzes the economic functions of the financial system. It identifies several key facts about financial structure: 1) banks are the primary source of external financing for businesses; 2) debt contracts are more common than equity; 3) financial intermediation reduces transaction costs. The chapter also discusses how asymmetric information between lenders and borrowers/shareholders and managers can lead to adverse selection and moral hazard problems. It analyzes various tools used to mitigate these issues, such as collateral, monitoring, and financial intermediation.