Since the work of Hans Selye (1973), the idea that stress can be detrimental to health has become common knowledge. Less commonly known is the evidence that stress may have detrimental effects on development (De Bellis, 2001; Gunnar & Vazquez, 2006; Heim, Plotsky, & Nemeroff, 2004). In this chapter, we describe what is known about the physiology of stress and its potential influence on young children. We then turn to studies of human development to examine the ways that stress is regulated early in life and the evidence that the stress system is responsive to adverse conditions during infancy and early childhood. Stress results when demands exceed immediately available resources (Lazarus & Folkman, 1984). These demands may be physical or psychological. Regardless, the imbalance in demands and resources requires that resources need to be found to meet the demands (Gunnar, 2000). These resources may be external, such as the help and support provided by parents and friends, or internal, such as a novel solution to a problem. Obtaining resources requires energy. Finding the energy needed for action and tuning the brain and body to meet the demands of the moment are the jobs of the stress system (Sapolsky, 1994). The stress system finds the energy we need to deal with immediate demands by putting future-oriented processes on hold. If there is an immediate threat to our survival, we do not need to put energy into fighting off a virus, digesting our lunch, or growing an extra inch.