 The core question underlying the existence of these two competing philosophical traditions is the extent to which something is formed or organized from “the beginning” or whether organization and form arise only over time. Nineteenth-century uses of the term “evolution” included a sense of unfolding of preexisting form, a sort of preformationism in contrast to the epigenesis of the day. Discussions of “evolution” and “epigenesis” can therefore be misleading in retrospect since the former term has assumed a meaning closer to the older meaning of epigenesis (Bowler 1975). Making things even more complicated, by the late nineteenth century “preformationism” really was more about various versions of predetermination or predelineation than preexistence of form as such. Furthermore some authors saw epigenesis or preformation as entirely internally directed, while others in each case allowed responses to the environment. In the background lie debates about the relative significance of predestination and free will, for persons, for organic beings, or even for the inorganic. In each case, it is important to keep in mind what the particular writer was saying and the arguments presented. Thus, discussions of epigenesis and preformation often bring in other ancillary questions and are difficult to separate from their contexts. This entry is an effort to extract what is centrally at issue and to focus on key contributions to the discussion.