Book excerpt: Why do we age? The answer to this question is critical to our ability to prevent and treat highly age-related diseases such as cancer and heart disease that now cause the deaths of most people in the developed world. This short book provides an overview of biological aging theories including history, current status, major scientific controversies, and implications for the future of medicine. Major topics include: human mortality as a function of age, aging mechanisms and processes, the programmed vs. non-programmed aging controversy, empirical evidence on aging, and the feasibility of anti-aging and regenerative medicine. Evolution theory is essential to aging theories. Theorists have been struggling for 150 years to explain how aging, deterioration, and consequent death fit with Darwin’s survival of the fittest concept. This book explains how continuing genetics discoveries have produced changes in the way we think about evolution that in turn lead to new thinking about the nature of aging.