The budding yeast Saccharomyces cerevisiae divides asymmetrically, giving rise to a mother cell and a smaller daughter cell. Individual mother cells produce a finite number of daughter cells before senescing, undergoing characteristic changes as they age such as a slower cell cycle and sterility. The average life span is fixed for a given strain, implying that yeast aging has a strong genetic component. Genes that determine yeast longevity have highlighted the importance of such processes as cAMP metabolism, epigenetic silencing, and genome stability. The recent finding that yeast aging is caused, in part, by the accumulation of circular rDNA molecules has unified many seemingly disparate observations.