Population Theory is a field of study that examines how populations grow, decline, and interact with their environment. It explores factors such as birth rates, death rates, and migration patterns to understand how these elements influence the size and structure of populations over time. Key figures in this field include Thomas Malthus, who proposed that populations grow exponentially while resources grow linearly, leading to potential shortages. Other theories, like the Demographic Transition Model, illustrate how societies transition from high birth and death rates to lower rates as they develop economically and socially.