In this chapter, we will first show that the subgame perfect Nash equilibrium cannot always eliminate some “sequentially unreasonable” Nash equilibria in extensive form games with imperfect information. As a remedy, we will then present two alternative refinements of the Nash equilibrium, known as the sequential equilibrium and the weak sequential equilibrium (or weak perfect Bayesian equilibrium). We will also give the relationship between these two refinements of the Nash equilibrium and their relations to the subgame perfect Nash equilibrium.