Play Without Regret Paolo Galeazzi Abstract: This thesis is centered on the issue of rational choice. Traditional decision-theoretic arguments aim at providing axiomatizations of different norms for decision making, and evaluate their rationality on the basis of the normative strength of their axiom systems. The approach taken in this dissertation is different. Instead of arguing in favor or against the normativity of some system of axioms, we decided to take a less well-trodden path: the study of the ecological rationality of different decision criteria. To this end, we extended the standard single-game models used in evolutionary game theory to include a multitude of different interactive decision problems. We consider the introduction of such a multi-game model, called the game of life, a principal contribution of this work in itself, in that it allows to lift the focus of the investigation from simple behavior to general behavior-generating mechanisms. The main results of this thesis concern the evolutionary competition between different ways of making choices in rich and complex environments. Classic decision criteria are compared from an ecological point of view, with respect to their evolutionary fitness, and regret-based principles prove to be especially beneficial in many (interactive) decision contexts.