Reproductive aging involves declines both in oocyte number and developmental capacity. Declining oocyte number alone cannot explain the manifestations of reproductive aging in women. We have proposed the Telomere Theory of Reproductive Aging to explain the complex phenotype found in oocytes from older women. Telomeres are TTAGGG repeats and associated proteins, which form loops at the ends of chromosomes to provide structural and genomic stability. Studies in mice and women show that telomere shortening in oocytes provides a parsimonious explanation for the effects of reproductive aging on oocyte quality. Measurement of polar body telomere length may predict oocyte quality in women undergoing ART.