Family planning programs, first undertaken nationally in the 1960s, have lowered fertility even when other factors such as lower death rates have worked to increase population growth. Socioeconomic conditions, such as higher literacy and greater urbanization, also influence fertility directly by increasing personal motivation to have smaller families. A dozen major cross-national studies have compared the effects of family planning programs and socioeconomic indicators on fertility; the most recent, by Mauldin and Lapham, analyzes fertility declines in nearly 100 developing countries from 1965-1980. The findings show that strong national family planning programs lower fertility independently of socioeconomic setting. The impact is greater, though, when socioeconomic variables are favorable. Researchers use 8 techniques to assess the family planning program's role in fertility decline: standardization or decomposition, trend analysis, standard couple-years of protection, reproductive process analysis, component projection, prevalence model, experimental design, and multivariate areal analysis. All 8 methods can be used to estimate births averted, but requires making problematic potential fertility assumptions. The article discusses the effectiveness of early programs in Taiwan, Singapore, Hong Kong, and South Korea. It then shows how different analysis techniques demonstrate birth rate decline for large scale programs in India, Indonesia, Thailand, China, Colombia, Mexico, Tunisia, and Mauritius. 3 examples of family planning programs with little effect, in Pakistan, Nepal, and Kenya, are also discussed. The program elements having the greatest impact on fertility are 1) adequacy of supervision at all levels, 2) staff competence, 3) mass media use, 4) postpartum programs, 5) services and supplies availability and accessibility, and 6) evaluation for program improvement. Over the last 2 decades, family planning programs have proved they can reduce fertility, and now researchers focus on why programs are successful. Government supported family planning programs of varying strength are currently under way in 127 countries; the key issue now is how the programs should be designed, implemented, and managed.