A DNA marker is a unit of measurable genetic variation that can be accessed reliably and repeatably. Genetic markers are carried by individual organisms through evolutionary time; it is these markers that geneticists use to address questions of population history and evolution. DNA markers are, in effect, molecular artefacts. The inferences that can be made with a particular marker or marker system depend in part on the rate at which variation accumulates. In general, functional genes are under strong selective constraint and vary slowly, whereas non-coding sequences and in particular repetitive sequences vary more rapidly. The different mutation rates of each system make it possible to examine population history at virtually any time depth.