This chapter discusses the definition and functions of money. It defines money as anything generally accepted as payment for goods, services, or debts. Money serves three main functions: as a medium of exchange, a unit of account, and a store of value. The chapter traces the evolution of payment systems from commodity money to modern forms like electronic payments and cryptocurrencies. It also examines how the money supply is measured using monetary aggregates like M1 and M2 that consider the liquidity of different assets.