Although substance use has been a worldwide problem at all levels of society since ancient times, recent attention has been focused on the use of legal and illegal substances by the pregnant woman. Almost all drugs taken by the pregnant woman are known to cross the placenta and have some effect on the fetus. This article reviews the effects of the drugs most frequently used by pregnant women in the United States--nicotine, alcohol, marijuana, opiates, and cocaine--on the fetus and neonate; when possible, information regarding long-term medical problems is included.