In a series of papers in the late 1980s, Amartya Sen claimed that about 100 million women were "missing," referring to the number of females who had died as a result of unequal access to resources in parts of the developing world. A subsequent debate has refined these estimates using different demographic techniques. In this paper, we review this debate, provide an update on the number of "missing women," and investigate the determinants of current trends in gender bias in mortality. We find that the number of "missing women" has increased in absolute terms, but fallen as a share of the number of women alive. There have been improvements for women's relative survival in most of South Asia and the Middle East, but deteriorations in China. Improving female education and employment opportunities has helped to reduce gender bias, while the increasing recourse to sex-selective abortions has worsened it.