Ontogeny (also ontogenesis or morphogenesis) describes the origin and the development of an organism from the fertilized egg to its mature form. Ontogeny is studied in developmental biology.

The idea that ontogeny recapitulates phylogeny, i.e. that the development of an organism exactly mirrors the evolutionary development of the species, is discredited today. Nevertheless, many connections between ontogeny and phylogeny can be observed and explained by evolutionary theory.