This chapter commences with an introduction to two types of formulations of economies with a measure space of economic agents. The aggregate demand relation which summarizes the total behavior of economic agents in perfectly competitive markets is introduced. Following this we proceed to discuss aggregation effects, that is, some specific effects on the total demands generated by aggregation or “summing” over individual demands. Two distinct sources of the effects are identified: one is a straightforward effect resulting from the mere fact of summing individual demands over a “large” population of economic agents, the other coming from the distribution of varying characteristics in the population. Aggregation effects will play an important role in discussing equilibria of economies in subsequent chapters. In the appendix, quotations from some of the most prominent classical economic theorists of the 19th and early 20th centuries are presented, in which they express their perspectives on the aggregation effects.