There is considerable controversy over the causes of the completed fertility transitions that occurred in most industrial countries from 1870 to 1930 and the “new” fertility transitions that are currently underway in the developing world. New data and empirical analyses of both historical and contemporary fertility declines have weakened the standard theory of the demographic transition, but none of the plethora of new theories of fertility change have emerged as hegemonie or as alternative guides to empirical research. The vast body of empirical evidence on the origins, speed, and correlates of fertility declines in different historical and geographical settings shows more diversity than a simple theory of fertility change would predict. The challenge for the field is to develop a common theoretical framework that will accommodate the diversity of historical paths from high to low fertility.