In the previous chapter, we mentioned that the Nash equilibrium has an interpretation in which each profile of equilibrium strategies reflects the conjectures of players about the other players’ strategies. In this chapter, we will deal with the question concerning whether or when a Nash equilibrium is robust (resistant) to some small mistakes (trembles) in these conjectures. Section 5.1 will define such a robust equilibrium, called the perfect equilibrium, and show how it can be calculated. We will refer to the perfect equilibrium as a refinement of the Nash equilibrium, as such an equilibrium will be more plausible than a Nash equilibrium that is not perfect. Section 5.2 will present an existence result for the perfect equilibrium in strategic form games, whereas Sects. 5.3 and 5.4 will, respectively, relate the perfect equilibrium to the Nash equilibrium and to the strategy profiles that are weakly undominated.