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This paper considers the robustness of optimal decisions in a model of monopoly and a model of competitive equilibrium in a securities market. Robustness, defined as payoffs being continuous with respect to perturbations in the underlying model, fails to hold for either model. Decision rules are made robust by appropriately "smoothing" the problem, and by decreasing the weight given to small details of belief that can cause large changes in optimal payoffs.

In a number of evolutionary models the presence of mutations, or random components of choice, serve to refine predictions of long-run behavior. We analyze the effects of mutation rates that vary because of the presence of imitation. A full characterization of long-run outcomes is provided for familiar coordination and congestion games, and also a number of other games not previously considered within the evolutionary framework. Our results are often quite distinct from those in the literature. We apply these tools to a series of economic applications, including market games, where imitation can explain excess volatility of prices, and location games, where it leads to greater uniformity in choices.