How is the brain shaped and refined by children's early social and emotionalexperiences? In this colloquium, I will focus on the development of children whohave endured environments marked by toxic levels of stress early in theirdevelopment. These children are known to be at increased risk for a variety ofhealth, academic, and social problems. Some of these problems appearimmediately, but others may not manifest themselves until much later indevelopment. I will highlight ways in which we can address central issues inhuman development by studying the quality and timing of children's socialexperiences. To do so, I will describe recent research involving children whohave experienced child abuse and neglect, children raised in poverty, childrenraised in institutional settings, children who have endured traumatic lifeexperiences, and typically developing children. Through these studies, I willhighlight new insights about the developmental processes underlying children'ssensitivity to their social environments as a way to understand the emergence ofboth adaptive and maladaptive human emotional behavior. Defining and specifyingways in which the environment creates long-term effects on brain and behaviorholds tremendous promise for improving the health and well-being ofchildren.