Prenatal stress, a function of severe maternal psychological or physiological stress, including trauma and illness, can influence the development of the fetus in profound ways, extending into childhood and adulthood. Maternal trauma, acute and chronic, physiological and emotional, can create an environment for the fetus that may disrupt development in the womb, potentially setting the infant on a path of maladaptive reactivity to stressors in the world beyond. Prenatal stress influences psychological systems, including attachment, affect, and stress regulation; physiological systems, including endocrine and cardiovascular systems; and even neuroanatomical structure. This article reviews some of the findings from animal models associated with this important line of research and indicates the critical nature of early intervention for families such as those described in this issue on the Project for Mothers, Infants, and Young Children of September 11, 2001.