Imagine two drivers arriving at an intersection at the same time. Without a traffic light, they must guess whether the other will go or stop — a dangerous guessing game. A traffic light acts as a mediator: it privately tells each driver what to do, but the signals are perfectly coordinated so that one gets “go” while the other gets “stop.” This chapter introduces correlated equilibrium, a broader idea that describes exactly this kind of coordination with a mediator. It often lets players reach better outcomes than they could with any independent Nash equilibrium.