Introduction, 629.—I. The basic model, 630.—II. Robustness, 638—III. Conclusion, 648.

This paper presents a simple, general framework for analyzing externalities in economies with incomplete markets and imperfect information. By identifying the pecuniary effects of these externalities that net out, the paper simplifies the problem of determining when tax interventions are Pareto improving. The approach indicates that such tax interventions almost always exist and that equilibria in situations of imperfect information are rarely constrained Pareto optima. It can also lead to simple tests, based on readily observable indicators of the efficacy of particular tax policies in situations involving adverse selection, signaling, moral hazard, incomplete contingent claims markets, and queue rationing equilibria.

We show that “spanning” does not imply stockholder unanimity if there is trading in the shares of firms. Each basis vector of the space spanned by all firms' output vectors can be treated like a composite commodity. If, in addition to spanning, firms act as price takers with respect to prices of composite commodities, then there is unanimity. We analyze the spanning assumption for the vector space of contingent claims generated by firms' choices of debt-equity ratios. We show that there is a strong relationship between the Modigliani-Miller theorem, spanning, and the existence of a complete set of markets.

I. An example, 473.—II. The generality of the Henry George Theorem, 477.—III. On using land rents as a measure of the benefits from public goods, 490.—IV. Competitive attainability of a Pareto optimal distribution of economic activity, 496.—V. Concluding comments, 498.

This introductory essay offers a brief guided tour of the main developments in the theory of implicit contracts, from its inception to the present. It is not intended as a survey but, rather, as an appraisal of the progress that has been made, the difficulties that remain, and as an outline of the microeconomic and macroeconomic issues that seem to invite additional work.