The years between 1900 and 1920 marked the formative era in the history of school social work. Social work programs were introduced into the schools by private organizations and community groups and were formed to prevent truancy and delinquency, to rehabilitate poor families through relief services, and to "Americanize" the foreign-born populations. Visiting teachers hired to staff these programs focused on environmental conditions rather than on the individual child. After World War I, a period of intense professionalization in school social work began, lasting from 1920 until 1965. Significant forces affecting school social work during the 1920s were the formation of the Commonwealth Fund of New York City and the mental hygiene movement. The expansion of federal relief and welfare programs after the depression allowed professional social workers to refocus efforts on case work and individual therapy; only after World War II did the services of social workers and counselors again flourish in the schools. In the era of federal intervention (1965-1972), local schools purchased social services from private agencies suggesting that, although schools had "housed" social service programs, such programs were never fully absorbed by the schools. (NRB)