Genetics is an important tool for identifying key molecular events that are involved in specifying biological functions. Genetic approaches have been used repeatedly to understand diverse biological phenomena: oncogenesis, development, and the cell cycle, but have only recently been applied to the analysis of organismic aging and senescence. The power of the genetic approach stems from two facts. First, genetic analyses allow the integration of phenomena that are analyzed at many levels of observation from the molecule to the intact organism, and second, genetics has the real power to reveal causality by factors that are not dependent upon the prejudice of the investigator. I discuss several areas where genetics has been fruitfully applied to the study of the aging processes: human genes identified by "segmental progeroid" mutations, neurological diseases of the elderly, the limited proliferative life span of human somatic cells in tissue culture, studies on the life span of the mouse, and genetic analysis of life span in shorter lived metazoans (Drosophila melanogaster and Caenorhabditis elegans), and the yeast Saccharomyces cerevisiae.