This chapter reviews the current understanding of the etiology and prevention of chronic diseases using a life course approach, emphasizing the lifelong influences on disease development. The global epidemic of obesity, diabetes, and other noncommunicable diseases (NCDs) is particularly prominent in developing and transitional economies, as well as in less affluent populations within developed countries. The epidemic is attributed to a lifetime of exposures and influences, including genetic predisposition and fetal programming. Prevention is the most cost-effective approach, involving three mutually reinforcing strategies throughout life, starting in the antenatal period. The chapter discusses the life course stages, from fetal development to old age, highlighting the importance of early life factors such as intrauterine growth retardation, premature delivery, and overnutrition. It also addresses the role of breastfeeding, postnatal growth, childhood and adolescent risk factors, and the impact of socioeconomic status on health. The chapter emphasizes the need for interventions to modify risk factors and promote healthy behaviors, particularly in older populations, to maximize quality of life and reduce the burden of chronic diseases.This chapter reviews the current understanding of the etiology and prevention of chronic diseases using a life course approach, emphasizing the lifelong influences on disease development. The global epidemic of obesity, diabetes, and other noncommunicable diseases (NCDs) is particularly prominent in developing and transitional economies, as well as in less affluent populations within developed countries. The epidemic is attributed to a lifetime of exposures and influences, including genetic predisposition and fetal programming. Prevention is the most cost-effective approach, involving three mutually reinforcing strategies throughout life, starting in the antenatal period. The chapter discusses the life course stages, from fetal development to old age, highlighting the importance of early life factors such as intrauterine growth retardation, premature delivery, and overnutrition. It also addresses the role of breastfeeding, postnatal growth, childhood and adolescent risk factors, and the impact of socioeconomic status on health. The chapter emphasizes the need for interventions to modify risk factors and promote healthy behaviors, particularly in older populations, to maximize quality of life and reduce the burden of chronic diseases.