Picture a farmer selling wheat at a busy grain terminal. She doesn’t set the price — she just looks at the big board, sees today’s number, and decides how many truckloads to deliver. This chapter is about markets where nobody can set the price. We’ll see how individual firms decide what to produce, when to keep going, and when to shut down. We’ll also see why these highly competitive markets tend to deliver exactly the right amount of stuff at the lowest possible cost.