This is the first in a series of articles in which we intend to critically review some of the currently used models in gerontology research and evaluate their contribution to advancing our understanding of the phenomenon of senescence. The major theories of aging are considered. We discuss what makes a model useful in general and for aging research in particular. We suggest criteria for the selection of paradigms for the study of aging. The criteria we suggest for identifying underlying mechanisms that lead to age related changes are: intraspecies universality, intrinsicality, progressiveness, and interspecies universality. The subsequent articles of this series shall consider the merits and possible drawbacks of some of the most commonly used models of the biology of aging: (a) the yeast Saccharomyces cerevisiae; (b) the nematode, Caenorhabditis elegans and the fruitfly, Drosophila melanogaster; (c) mammalian cells in culture and telomerase model; (d) mitochondria and aging; (e) progeroid syndromes; (f) in vivo studies with laboratory rodent strains; and (g) plant senescence.