Aging in humans (and animals) can be seen as either an inevitable process of wear and tear or as an inherent biological program by which the lifespan of each species is more or less predetermined. Recent research has shown that DNA methylation, an epigenetic modification which alters how DNA is read and expressed without altering the underlying sequence, can show age-related changes. A sub-set of these modifications are so accurate that chronological age in humans can be predicted +/- 3.6 years from any tissue or fluid in the body. This is by far the best biomarker of age available and is referred to as the epigenetic clock. Interestingly, analysis of DNA methylation can also provide information on biological age, which is a measure of how well your body functions compared to your chronological age. For instance, people suffering from fatty liver disease have a faster ticking clock, while centenarians have a slower clock.