In strategic form games, the strong or weak dominant equilibrium usually does not exist and even the iterative elimination of strongly or weakly dominated strategies cannot always predict a unique strategy profile as an equilibrium. However, as Sect. 3.1 of this chapter will make it clear, in any strategic form game the players can always calculate their strategies that are best response to a strategy profile of their opponents. Using the best-response strategies, we will define in Sect. 3.2 the sets of strategies of each player that can be predicted to be rationally played, or simply “rationalizable”.