We discuss the background concepts which lead to this issue of Experimental Gerontology. On one hand, genetic and molecular studies of short-lived worms, flies, and mice are yielding remarkable discoveries on gene systems that regulate the life span. On the other hand, little is known about the nature of aging in other vertebrates, with life spans extending into the human range or beyond the record 122y human life span, which may have aging processes that are so slow as to be 'negligible'. We point out that organisms with these vastly different life spans have essentially identical cells within an evolutionary group and that the cellular tool kit that existed by 600 million years ago allowed the evolution of life spans ranging up to one million-fold difference in length. The possibility of negligible senescence has not been widely discussed, and may be in conflict with mathematical deductions from population genetics theory. We propose minimal criteria for the lack of senescence: (1) no observable increase in age-specific mortality rate or decrease in reproduction rate after sexual maturity; and (2) no observable age-related decline in physiological capacity or disease resistance. We also introduce some of the species discussed in subsequent chapters which are unfamiliar models to most biomedical researchers. Copyright © 2001 Elsevier Science Inc.