This technical note sets the groundwork for a module on macroeconomics by introducing concepts and definitions that will be revisited in subsequent technical notes. The note begins by introducing national income accounting and the components of gross domestic product (GDP), and by discussing why economists care about GDP. It then discusses how the determinants of the components of GDP interact on the market, and how we can predict GDP and interest rates through the lens of a model?a framework that maps assumptions into outcomes. The note introduces students to basic economic modeling by walking them through examples of how to determine endogenous variables from systems of equations and exogenous variables. The note concludes by discussing the role of assumptions in macroeconomics and by listing some of the assumptions that we will maintain in subsequent technical notes.