Documentation that family planning programs are capable of inducing major and rapid decline in fertility is still quite scarce. This monograph tests hypotheses linking family planning programs to changes in fertility by assembling data for four specific LDCs. The first chapter sets out a methodology for measuring the impact of family planning programs on birth rates. Two packaged computer programs (PROJTARG and TABRAP) are used for calculating the contraceptive protection (number of new acceptors) that must have been attained in the test area in order to have resulted in the observed fertility rate reduction. The proportion of protection provided by inside-program as compared with outside-program sources is then estimated to the maximum extent possible. With this information, the net proportion of total contraceptive protection provided by the organized family planning program can be determined under differing assumptions about the substitutibility of contraceptives (the number of acceptors who would have used other methods if the formal program had not existed). This model is then applied to four countries -- Colombia (treated on both a national and a state-by-state level), Thailand, Indonesia, and Korea. These four countries have all sponsored vigorous family planning programs since at least 1970. Each of these family planning programs is shown to have been the major source of contraceptive protection by which observed fertility declines have been achieved. Presentation of the Colombian example includes a detailed discussion of the use of various demographic techniques to obtain consistent estimates of fertility levels and contraceptive usage. In the Indonesian case, the impact of the family planning program is not judged by comparing contraception provided by the program against required contraception, as reliable data needed for the computer programs was not available. A births-averted procedure was used instead. The studies in the Thai and Korean cases also suggest that government support for family planning programs is crucial. Sizable (15-25 items) reference bibliographies are included after each chapter.