Thesis advisor: Fabio GhironiMy dissertation develops a set of tools for introducing heterogeneity into economic models in an analytically tractable way. Many models use the representative agent framework, which greatly simplifies macroeconomic aggregation but abstracts from the heterogeneity we see in the real world. In my research, I move away from the representative agent framework in two key ways. First, my work in international macroeconomics incorporates heterogeneity via idiosyncratic shocks across countries. Second, my work on financial frictions employs asymmetric information between lenders and borrowers. In both of these areas, my goal is to examine the implications of heterogeneity in the most tractable way possible. Cruci...