Chapter 3 turns to the determinants of the supply of goods and services and to the way in which the “forces” of demand and supply determine prices and the quantities exchanged. Via the use of simple models, economists explain generalizations concerning how the quantities of goods and services supplied respond to prices. The accounts of demand in chapter 2 and supply in this chapter take prices as given, and additional modeling is needed to explain how supply and demand are equilibrated and what properties market equilibria possess. This chapter pulls together the discussions of the first three chapters to offer a general sketch of the causal structure and basic principles of mainstream economics. It takes issue with the view, which used to be dominant, that general equilibrium theory is the fundamental theory of contemporary economics. What I call “equilibrium theory,” not general equilibrium theory, is fundamental.