I n this essay, we discuss the emerging literature in macroeconomics that combines heterogeneous agent models, nominal rigidities, and aggregate shocks. This literature opens the door to the analysis of distributional issues, economic fluctuations, and stabilization policies-all within the same framework. Quantitative macroeconomic models have integrated heterogeneous agents and incomplete markets for nearly three decades, but they have been mainly used for the investigation of consumption and saving behavior, inequality, redistributive policies, economic mobility, and other cross-sectional phenomena. Representative agent models have remained the benchmark in the study of aggregate fluctuations (for reasons we will discuss later). However, the Great Recession bluntly exposed the shortcomings of a representative-agent approach to business cycle analysis. A broadly shared interpretation of the Great Recession places its origins in housing and credit markets. The collapse in house prices affected households differently, depending on the composition of their balance sheets. The extent to which this