The fertility decline in Thailand is the 3rd largest fertility decline behind only South Korea and China. Thailands total fertility rate of 6.4 during the 1st half of the 1960s was quite typical of large Third World countries; by the early 1980s total fertility in Thailand had fallen to 3.6. Thailand is an especially intriguing case because of the rapidity and pervasiveness that have characterized the fertility decline there and because reproductive change has occurred during a period when the country is still predominantly rural and agrarian. The significance of Thailand as a case study of reproductive change in the Third World is further enhanced by the fact that at an early stage of the fertility decline an official policy and program were instituted to encourage and facilitate the practice of contraception with the explicit goal of reducing the population growth rate through lowering fertility. Although Thailand is not among the least developed of the Third World countries neither does it rank particularly high with regard to many conventional indices of socioeconomic development. Chapter 2 discusses sources and methods and provides socioeconomic and related background information for Thailand. Chapters 3 and 4 review recent trends in fertility and family size preferences. The quantitative evidence concerning the proximate determinants of reproductive change is examined in chapter 5 which includes an assessment of the relative contribution of several key proximate determinants to the recent levels and changes in Thai fertility. The results clearly show that increased contraceptive use is the main proximate determinant accounting for Thailands fertility decline. Chapter 6 also focuses on the proximate determinants but is based on qualitative evidence and includes a discussion of evidence pointing to a substantial latent demand for lower fertility even before fertility went down. The relations between societal change and reduced family size are the topic of chapter 7. Chapter 8 focuses on aspects of the Thai culture that are particularly relevant to understanding reproductive behavior and attitudes. Chapter 9 explores the channels through which the means of fertility reduction spread through Thai society. The final chapter summarizes basic arguments regarding the causes and conditions behind Thailands rapid fertility decline.