Integration of noninvasive, biological measures into behavioral research has increased, but the interpretation of biobehavioral findings in relation to developmental outcomes is rarely straightforward. This commentary highlights the need for specific, theoretically derived hypotheses, multiple measures of behavioral and biological processes, and analytical strategies aimed at explaining interindividual differences in intraindividual change. It is suggested here that the next phase of biosocial research needs to move beyond description and toward development of mid-level theories that will enable researchers to specify, test, and refine hypotheses of how biobehavioral processes interact with social-contextual factors to influence development. These mid-level biosocial models will be necessary to determine whether individual differences in children's adrenocortical activity confer risk or resilience because of early or cumulative exposure to nonparental care.