The study of demography must begin with an understanding of the three sources of population changes: fertility, mortality, and migration. This paper leads prospective demographers--or anyone interested in population--through the dynamics of these three variables, introducing them to the forces that cause populations to grow or decline, and that determine the age, sex, or ethnic composition of a society. The effects of fertility, mortality, and migration in changing the geographic distribution of people in the world and in the United States are studied. The unprecedented global population growth that has occurred within the past few centuries is noted, as well as the prospects for future growth. Finally, the many critical population-related issues facing the world today are surveyed. A list of discussion questions concludes this document. (Author/DB)