Though it has been ten years since the Great Recession, the comprehensive macroeconomic models in use at central banks, government agencies, and other large financial institutions are not noticeably improved from a decade ago. Conversations with leaders of those institutions point to two fundamental flaws in traditional models, namely, the assumptions about representative agents and about rational expectations. These imply not only that the economy evolves as if there is only one consumer and only one firm but also that the consumer and the firm make optimal decisions based on predictions that are realized. Why are macroeconomists so reluctant to give up these stultifying assumptions? Because as hard as it is to run models with those assumptions, it is nearly impossible to compute much without them. Chris Carroll of Johns Hopkins University wants to fix this situation. While serving as chief economist at the Consumer Financial Protection Bureau (CFPB), he started constructing an open source computational tool kit for macroeconomists that can specifically handle non-rational heterogeneous agents. The platform, the Heterogeneous Agents Resources Kit (HARK), is capable of modeling how microeconomic interactions among heterogeneous agents can lead to macroeconomic outcomes different from those predicted by traditional techniques. It is also possible to assign less-than-rational behaviors—such as hyperbolic discounting, anchoring, or herding—to parts of the population. Running simulations under those circumstances can reveal phenomena that traditional models can neither explain nor even generate. This grant provides three years of support to Carroll as he further expands and develops HARK and creates tools to facilitate its use.