A virtual textbook on what the growing knowledge of biochemistry, genetics, and molecular biology is revealing about the basic mechanisms of aging. Clark, professor emeritus of immunology at Univ. of Calif., Los Angeles, whose previous work, The New Healers: Molecular Medicine in the Twenty-First Century (1997), seriously challenged readers not trained in these sciences, has returned to the classroom for yet another dense lecture, complete with graphs, tables, and diagrams. With technical expertise, he describes in detail how the process of aging, or senescence, takes place at the level of individual cells and what is known about the internal regulation of that process by our genes. He relates how research into various genetic disorders that mimic the human aging process, such as Hutchinson-Gilford progeria and Werner’s syndrome, is providing clues to the involvement of genes in the aging process; and he dismisses claims that rejuvenation or immortality can be achieved through the manipulation of telomeres, those segments of DNA at the tip ends of chromosomes that have been called the cell’s internal timekeepers. The effects of caloric restriction on lifespan are considered, as is the role of antioxidants in reducing the risk of deadly cancers and cardiovascular disease. What is clear from Clark’s text is that more is known about how and why we age than has ever been known before, and that with the coming completion of the Human Genome Project, knowledge of the genes involved in aging will greatly expand. In his final chapter, which is the book’s least technical and most thought-provoking, Clark considers briefly what the impact on society will be as that knowledge is applied and leads to a longer, healthier average human lifespan. A scientist’s careful, unsensational account of the current status of research into aging that requires from the reader a level of commitment well beyond mere curiosity.