This chapter analyses the interaction between firms with market power—a situation we call oligopoly. Oligopoly theory consists of a range of specialised models, and the exercises explore some variations of the material covered in the textbook. What happens if we change the number of firms in the market, or introduce differences in costs or demand? Can advertising your product also benefit your competitor? And what are the implications for economic efficiency if a foreign company enters the market? These exercises offer a deeper understanding of the theory by applying it to a range of interesting questions.