Women in Japan and elsewhere are increasingly delaying childbearing. The percentage of Japanese women giving birth to their first child at the age of 30 and above has increased from 28.9% in 1980 to 58.9% in 2009. As a result, women seeking to conceive one or more children increasingly do so when the effects of reproductive aging first become noticeable. Female fertility begins to decrease after 30 years of age, falls markedly after the age of 35 in association with an increased risk of complications during pregnancy and of chromosomal abnormalities in the offspring, and most women become infertile after the age of 40. In parallel, the number of assisted reproductive technology (ART) treatment cycles in Japan is steadily increasing, from 37,455 in 1997 to 190,613 in 2008, according to a survey by the Japan Society of Obstetrics and Gynecology. Because of the high emotional and social toll of infertility on couples and the demographic consequences of a decrease in fecundity, it is imperative to better understand the biology of reproductive aging. Here we review current knowledge about the decline of female fertility during aging and discuss the implications for infertility treatments.