Preterm birth, with its disproportionate contribution to perinatal mortality and morbidity, remains the number one problem in perinatal medicine. The fact that the preterm birth rate has actually risen is particularly disconcerting, given the enormous efforts and resources that have been expended to defeat this problem over the past two decades. However, progress actually has been made toward understanding the pathophysiology of preterm labor and in developing effective means of prevention, early diagnosis, and therapy.