A fundamental question in the developmental social and behavioral sciences concerns the specific necessary and sufficient infancy and early childhood experiences that will lead to typical human development and mental wellbeing. Since the opportunity to change early experience within an experimental study is limited, one approach is to observe the development and long-term outcomes of children who are reared in deficient early environments. This presentation focuses on a quasi-experimental intervention study of the role of early social-emotional experience in the development of birth-4-year-old children living in three orphanages in St. Petersburg, RF. One orphanage received training aimed at improving the sensitivity and responsiveness of caregivers, plus structural changes to increase caregiving stability and consistency. Another orphanage received training only, and a third—treatment as usual. The results show improvement in children’s physical, mental, and social-emotional development in the training plus structural changes environment. These results substantiate the importance of early social-emotional experience for the development of children, and provide a rationale for making similar improvements in other institutions, foster care and non-residential care environments.