Multiple definitions of the concept of epigenesis exist in the developmental biology and psychology literatures. The descriptive use of the term is generally associated with qualitatively discontinuous change, as represented by the increasing differentiation of behavioral attributes involved in the orthogenetic principle. These descriptions are accounted for by two types of explanation. Predetermined epigenetic conceptions stress that the bases of the changes characterizing development arise from nature variables, and that nurture variables serve only to facilitate or inhibit preformed, intrinsic trends. Probabilistic epigenetic conceptions stress that reciprocal interactions among nature and nurture variables provide the bases of behavior change. The second conception of epigenesis is seen as the more scientifically useful one because it does not exclude from explanatory research consideration of biology-context interdependencies. The descriptive and explanatory issues involved in understanding the concept of epigenesis are discussed in relation to points raised in Kitchener’s (1978) recent paper on epigenesis, a presentation that did not adequately distinguish between the various uses of the concept and their implications for science.