We have seen how Nash equilibrium and subgame perfection help us predict play in games that unfold over time. But many real-world situations involve hidden moves — a player must act without knowing exactly what happened earlier. This chapter gives you a tool that handles that hidden history: sequential equilibrium. It pairs a sensible plan of play with a consistent guess about the past, and then we push the idea further with the intuitive criterion, a refinement that helps us pick out even more believable outcomes in signaling games.