InEvolutionary stable strategy this chapter, we will consider robustness of an equilibrium from an evolutionary perspective. Briefly, we will be dealing with the question whether or when a large population of players that is randomly matched as pairs to play a symmetric two-person strategic form game is resistant against the strategies of mutants that try to invade the population. The answer will yield a concept called evolutionary stable strategies. In this chapter, we will also present the relation between this concept and several notions of equilibrium presented in earlier chapters.