Important research advances on the psychobiological mechanisms underlying individual differences for stress reactivity have resulted from research in animal models. Animal research has also shed light on the psychobiological correlates for stress reactivity in humans, providing candidate neurochemical mechanisms for human research and suggesting general theoretical models of psychological functioning across species. In this article, the basic neurochemical elements underlying stress reactivity are described. In addition, the probabilistic epigenesis theoretical model is used to introduce the necessary developmental and genetic context for the understanding of the psychobiology of individual differences in stress reactivity. Finally, some previous research and potential avenues of research in posttraumatic stress disorder are described using the concepts that emerged from animal research.