Today's awareness of world population problems owes much to the work of private organizations. The private sector has traditionally taken the lead in this field, pressing governments of both developed and developing countries to adopt appropriate policies and to extend family planning services and education nationwide. While each organization has its own distinct program, most population organizations work cooperatively to support effective actions and new initiatives in such varied fields as family planning services and communication, contraceptive and demographic research, and political action. Population organizations worldwide now number literally in the hundreds. Therefore, the list of 40 organizations provided (with descriptions of their activities) is highly selective, including only major private organizations with a primary focus on population trends, family planning programs, and reproductive rights. Additional selection criteria include publisher of influential publications, funder or implementing agency of family planning programs, and extensive outreach or influence (implied by an annual budget of $350,000 or more). A supplementary listing of major professional membership organizations in the population field, multilateral organizations and government agencies providing population assistance, U.S. foundations providing funding for population programs, and major U.S. (university-based) population study and research centers is also provided. (Author/JN)