Heterochrony refers to any change in the developmental timing of an organism's growth that results in changes between ancestor and descendant species; or between different members of a species (e.g. sexual dimorphism). In essence, it is the modern take on the old nineteenth century "Biogenetic Law" which held that ontogeny (the developmental history of a species) recapitulated phylogeny (the evolutionary history of a species). This generalisation gained wide acceptance, and still rings true, not least because of some striking features of human embryological development. For instance, human embryos possess gill slits at a certain stage: it seems as though the embryo passes through various stages corresponding to stages of progress from "lower" evolutionary forms (eg. protists) to "higher" forms (eg. vertebrates). But human embryos never pass through adult forms of putative ancestors: we may have gill slits, but we never actually resemble adult fish.

Something like "ontogeny recapitulates phylogeny" does occur as a result of evolutionary changes in developmental timing. For instance, descendants may mature precociously relative to ancestors, causing morphological differences. Whenever such a shift in developmental timing occurs, it is called heterochrony. Heterochrony lies behind the relatively rare cases of "ontogeny recapitulating phylogeny". More importantly, heterochrony can have other evolutionary effects, and recognising its action has important consequences for debates regarding the roles of adaptation and developmental constraints in evolution, as well as the nature of evolutionary dynamics.