The prevalence of smoking during pregnancy varies markedly across countries. In many industrialized countries, prevalence rates appear to have peaked and begun to decline, whereas in other countries smoking is becoming increasingly common among young women. Randomized controlled trials have shown that smoking interventions during pregnancy have had limited success. Smoking during pregnancy is in many countries recognized as the most important preventable risk factor for an unsuccessful pregnancy outcome. Smoking is causally associated with fetal growth restriction, and increasing evidence also suggests that smoking may cause stillbirth, preterm birth, placental abruption, and possibly also sudden infant death syndrome. Smoking during pregnancy also is generally associated with increased risks of spontaneous abortions, ectopic pregnancies, and placenta previa and may increase risks of behavioral disorders in childhood. Smoking during pregnancy will continue to be an important risk factor for maternal and fetal outcomes during pregnancy.