0. Introduction

The stage on which this book's plots are set is a model called an economy. The model captures situations in which each agent in a society chooses an alternative and there exists a fundamental tension between the agents' personal desires and society-wide feasibility constraints. The chapter includes many examples of economies.

A candidate for an equilibrium includes two components:

(i) A profile of choices - one choice for each agent.

(ii) A specification of certain parameters that systematically influence either agents' choice problems or their preference relations.

In equilibrium, agents make individually optimal choices, and the parameters restrict their choice sets to be compatible, in the sense that the resulting profile of choices is feasible.

We provide an overview of the equilibrium concepts we study in the book. They are classified into two groups. In the choice group, each agent's choice set depends on a price-like equilibrium parameter, but not on the equilibrium profile of choices. In the deviation group, an equilibrium is a profile of choices which is immune to any single agent's deviation from his prescribed alternative to any other alternative in a set determined by the equilibrium parameters.