We love the idea of a perfectly rational player who never makes a mistake. But in real life, fingers slip, attention wavers, and a button that should say “Cooperate” sometimes gets pressed as “Defect” by accident. This chapter asks a simple but deep question: if we take those tiny, inevitable mistakes seriously, which Nash equilibria still make sense? The answer leads us to a neat idea called trembling-hand perfect equilibrium. It’s a way to pick out equilibria that stay stable even when players have a tiny chance of making a mistake.