Robustness is a prominent concept in technical sciences and engineering. It has also been recognized as an important principle in evolutionary biology. In this chapter, it is proposed that the term “robustness” be used to characterize the extent to which a natural or artifi cial system can maintain its function when facing perturbation and that this concept is relevant in considerations of Darwinian decision theory. Situations in which the action of natural selection is liable to lead to the evolution of robust behavioral strategies are highlighted along with some psychological mechanisms that might lead to robust decision-making processes. Robustness describes a property of a system varying on a continuous scale rather than existing as a dichotomous feature. Degree of robustness depends on the details of the interaction of system characteristics and environmental contingencies, as well as the specifi c types and extents of perturbations to which the system may be subjected. A system can be robust in one domain while remaining highly vulnerable to perturbations in others. As defi ned here, robustness is related to, yet distinct from, fl exibility and optimality. The sorts of environmental variation, and hence perturbations, that an organism or technology is liable to face are described, as is the cost-benefi t trade-off of robustness. Finally, the robustness of decision making at the level of social groups is considered.