The article presents a theory of epidemiologic transition, which examines the complex changes in health and disease patterns and their interactions with demographic, economic, and sociologic determinants. This theory, developed by the author, builds upon the limitations of demographic transition theory and emphasizes the need for a comprehensive approach to population dynamics. The theory is based on the observation that patterns of health and disease are integral components of population change, and that epidemiologic knowledge can help predict and understand these changes. The theory outlines three basic models of the epidemiologic transition: the classical or western model, the accelerated model, and the contemporary or delayed model. The classical model describes the gradual transition from high mortality and fertility to low mortality and fertility in western societies. The accelerated model, exemplified by Japan, shows a faster transition. The contemporary model, seen in many developing countries, reflects a recent and ongoing transition. The theory highlights the role of mortality in population dynamics, noting that mortality is a fundamental factor in population change. It also discusses the shifts in disease patterns, from pandemics of infection to degenerative and man-made diseases. The theory further explores the relative risks of mortality by age and sex, showing how improvements in health and survival affect fertility and population growth. The theory also examines the interactions between epidemiologic and socioeconomic transitions, emphasizing how improvements in mortality and health contribute to economic productivity and development. The article concludes by emphasizing the importance of understanding these transitions for effective population policy-making and the need for a multidisciplinary approach to population theory.The article presents a theory of epidemiologic transition, which examines the complex changes in health and disease patterns and their interactions with demographic, economic, and sociologic determinants. This theory, developed by the author, builds upon the limitations of demographic transition theory and emphasizes the need for a comprehensive approach to population dynamics. The theory is based on the observation that patterns of health and disease are integral components of population change, and that epidemiologic knowledge can help predict and understand these changes. The theory outlines three basic models of the epidemiologic transition: the classical or western model, the accelerated model, and the contemporary or delayed model. The classical model describes the gradual transition from high mortality and fertility to low mortality and fertility in western societies. The accelerated model, exemplified by Japan, shows a faster transition. The contemporary model, seen in many developing countries, reflects a recent and ongoing transition. The theory highlights the role of mortality in population dynamics, noting that mortality is a fundamental factor in population change. It also discusses the shifts in disease patterns, from pandemics of infection to degenerative and man-made diseases. The theory further explores the relative risks of mortality by age and sex, showing how improvements in health and survival affect fertility and population growth. The theory also examines the interactions between epidemiologic and socioeconomic transitions, emphasizing how improvements in mortality and health contribute to economic productivity and development. The article concludes by emphasizing the importance of understanding these transitions for effective population policy-making and the need for a multidisciplinary approach to population theory.