Sensitivity to environmental context has been of interest for many years, but the nature of individual differences in environmental sensitivity has become of particular focus over the past 2 decades. What is particularly uncertain are the neural variables and processes that mediate the effects of environment on developmental outcomes. Accordingly, in this dissertation, a neurobehavioral model of sensitivity to the environment is proposed and tested in a large sample of human subjects. First, the different patterns of environmental sensitivity are defined to identify the significant factors involved in the manifestation of these patterns. Second, the mechanisms of neurobiological reactivity underlying variation in sensitivity to the environment are proposed by providing an organizing threshold model of elicitation of emotional neurobiology by environmental context. Third, developmental predictions of the model are explored, namely that reactivity of emotional systems will reflect endogenous sensitivities and environmental history. Finally, an empirical investigation of this model is presented. The sensitivity of three emotional systems (social bonding, incentive approach, and stress reactivity) was assessed by exposing participants (N = 398) to stimuli that elicit their activity. In particular, 1) soft touch is a potent social reward activating the social bonding system; 2) anticipation of winning a monetary reward activates the incentive motivation system; and 3) uncertainty of an aversive noise activates the stress response. First, the magnitude of soft touch, monetary reward and uncertainty was varied (i.e., number of brush strokes, amount of monetary reward, and degree of uncertainty), and participants reported immediate emotional feelings in response to each intensity level. Second, participants engaged in an associative conditioning procedure in which each stimulus was paired with a neutral context. The degree of conditioning represents how deeply the emotional experiences were processed to affect learning, and hence, plasticity. Results show that momentary responses to each emotional stimulus strongly predicted degree of associative conditioning and that emotional sensitivities were related in expected ways to early environmental experience and personality traits. These findings suggest that the proposed emotional systems are mechanisms of adaptation to the environment, and that the function of these systems reflect environmental history.