This book analyzes the existence of equilibria in economies having a measured space of agents and a continuum of agents and commodities. Excessive homogeneity with respect to agent productivity leads to instability and non-uniqueness of a given stationary state and the indeterminacy of the corresponding stationary state equilibrium. Sufficient heterogeneity leads to global saddle-path stability, uniqueness of a given stationary state and the global uniqueness of the corresponding equilibrium.