The classic Arrow–Debreu framework requires a very large number of specific securities to reach Pareto optimality. The present paper shows that financial intermediation can play an important role in maintaining a more parsimonious market framework while still obtaining Pareto optimality. In the framework developed, the aggregate risk components of individual risks are exchanged through a highly reduced set of nonspecific securities, while the idiosyncratic risk components are insured through financial intermediation. Reaching Pareto optimality does not rest on a Law of Large Numbers approximation. The role of financial intermediation is complementary to the role of security derivatives and dynamic trading.