The prevalence of smoking during pregnancy is decreasing in some, but not all, developed countries, as the prevalence of overweight is increasing rapidly in most countries. During pregnancy, smoking increases the risk of placental abruption, but reduces the risk of preeclampsia, while overweight increases the risks of gestational diabetes, preeclampsia, and cesarean delivery. Both smoking and overweight are associated with dose-dependent increases in risks of stillbirth and very preterm birth (< 32 weeks). From a public health perspective, smoking and overweight today probably represent the most important modifiable risk factors for pregnancy complications and adverse pregnancy outcomes.