The notion that phenotypic traits, including behavior, can be predetermined has slowly given way in biology and psychology over the last two decades. This shift in thinking is due in large part to the growing evidence for the fundamental role of developmental processes in the generation of the stability and variations in phenotype that researchers in developmental and evolutionary sciences seek to understand. Here I review the tenets of a metatheoretical model of development called probabilistic epigenesis (PE) and explore its implications for furthering our understanding of developmental and evolutionary processes. The PE framework emphasizes the reciprocity of influences within and between levels of an organism's developmental manifold (genetic activity, neural activity, behavior, and the physical, social, and cultural influences of the external environment) and the ubiquity of gene-environment interaction in the realization of all phenotypes.