Why do some markets work beautifully, giving us cheap products and plenty of choice, while others feel slow, expensive, and frustrating? The answer often comes down to how much competition a market really faces. In this chapter, we will build a simple but powerful toolkit to measure how well a market is working — not by guessing, but by adding up the benefits to buyers and sellers. Then we will use that toolkit to compare two extreme cases: fierce competition and a market controlled by a single seller.