Aging involves multiple harmful biological events that accumulate in different tissues over time and gradually reduce an organism’s state of maintenance and function. Calendar time, however, serves as an imperfect measurement of the physiological processes involved in aging. We all know individuals who are the same chronological age but appear to be very different when it comes to physiological age. Rather than counting years — or gray hairs, for that matter — modern gerontologists turn to biological markers, or biomarkers, of aging. These physiological parameters indicate an individual’s functional level and some biomarkers, such as insulin levels, correlate with mortality. The presence of such biomarkers depends indirectly on patterns of gene expression, which are induced by a variety of internal or external stimuli.