Population change can be interpreted as the result of the continuous confrontation and adaptation between the forces of constraint and the forces of choice. Forces of choice are the ability to modulate and control behaviors that have demographic consequences, such as entering into a reproductive union; having children; protecting and enhancing health with adequate nutrition, housing, and clothing; moving and migrating from one place to another. Modern demography has been characterized by an acceleration with a variety of geographical patterns, and this variety increases the smaller the scale of analysis. This chapter outlines the nature of the demographic systems prevailing in different parts of the world in the eighteenth century. It presents the factors that determine a change or a transformation of a demographic system, therefore affecting population development. To define demographic transition as the process that has reduced mortality and fertility from the high pre-nineteenth-century levels to the low ones that prevail nowadays in Europe, America, and East Asia.