The assumption that there exists a firm relationship between the preferences of the economic unit and its behavior (choice from the set of the feasible alternatives it faces) is very common in economic theory. In fact, the exclusive working supposition in the analysis of the economic unit's behavior is that it can be assigned a system of preferences, such that its choice is the best alternative according to that system of preferences. It should be noted that under this assumption, which is often called the rationality principle, the system of preferences assigned to the economic unit is conceived as a device (theoretical construction) that enables explanation and prediction of its behavior. The existence of such a tight relationship between preferences and choice is not necessary. The system of preferences need not be well defined, and still a meaningful choice may exist. It is also possible that the system of preferences is well defined, but is inconsistent with the observed choice of the economic unit. And it is also possible that the notion of preferences is questionable or even meaningless as, for example, when it is assigned to a group of individuals who face the task of making a collective choice. This chapter is devoted to a formal presentation of the concepts of preference and choice and to a clarification of the relationship between them.