This book contributes substantively to the current state of the art of macroeconomic modeling by providing a method for modeling large collections of heterogeneous agents subject to nonpairwise externality called field effects, i.e. feedback of aggregate effects on individual agents or agents using state-dependent strategies. Adopting a level of microeconomic description that keeps track of compositions of fractions of agents by "types" or "strategies", time evolution of the microeconomic states is described by (backward) Chapman-Kolmogorov equations.