Links between aging and epigenetics have been revealed by bio-mathematicians. Methylation of cytosine, which is a characteristic of the epigenome, varies with age on some ADN loci, increasing or decreasing. From an analysis of the methylome, algorithms giving an "epigenetic age" were obtained, strongly correlated with the age. Surprisingly, this approach could be applied consistently to different tissues or unpurified cells. It was successfully applied to tissues of 185 mammalian species. The epigenetic age of embryonic pluripotent stem cells is nearly zero and it decreases to "ground zero" during gastrulation. The average methylation curve as a function of age allows discrimination between slowly or rapidly aging individuals. At the present time, more than 10 different epigenetic clocks have been proposed for medical applications. The localization of aging-sensitive CpG pairs on the genome revealed networks of "co-methylation", involved in different functions such as regulation of morphogenesis or cell differentiation. From these studies, aging appears as a continuous process, with the epigenetic clock starting to "tick" in the embryo.