In this entry, we focus on the ex interim stage of decision making. This is in line with much of the literature on the epistemic foundations of game theory as it allows for a straightforward assessment of the agents’ rationality given their expectations about how their opponents will choose. Focusing on the ex interim stage does raise some interesting questions about possible correlations between a player’s strategy choice, what Stalnaker (1999) calls “active knowledge”, and her information about the choices of others, her “passive knowledge” (idem). The question of how a player should react, that is eventually revise her decision, upon learning that she did not choose “rationally” is an interesting and important one, but we do not discuss it in the entry. Note that this question is different from the one of how agents should revise their beliefs upon learning that others did not choose rationally. This second question is very relevant in games in which players choose sequentially, and will be addressed in Section 4.2.3.