Over the past quarter century the sex ratio at birth (SRB) has risen above natural levels in a number of countries, mostly in Asia. This rise has been made possible in populations with strong son preference by the increasing availability of safe, effective, and inexpensive technologies to determine the sex of a fetus and to end unwanted pregnancies. This article documents levels and trends in the sex ratio at birth, in preferences for male offspring (using information on desired number of girls and boys), and in the implementation of these preferences. DHS surveys from 61 countries in Africa, Asia, and Latin America and for Indian states are the main source of data. A comparison of desired with actual SRBs finds large gaps in most populations, implying a substantial pent-up demand for male offspring and the technology to implement this preference. Two types of actions to implement preferences are considered: the practice of contraception to stop childbearing after the desired number of sons has been born and the use of sex-selective abortion to avoid female births. The second part of the article discusses factors that could influence the SRB, including the promotion of gender equality, and the implications of these factors for future trends.