An understanding of the epidemiology of alcohol and drug use in young women is important for three main reasons: (1) to appreciate that substance use, misuse, harmful use and dependence are associated with considerable mortality and physical and psychological morbidity; (2) to understand the nature and extent of these problems and the likely impact on the fetus, neonate and infant through childhood to adolescence; and (3) to utilize this information as part of a needs assessment to develop effective services, which detect problems and deliver appropriate interventions. Although abstention rates are consistently higher among women than men in general, substance misuse is increasing in young women. Simultaneously, there is great variability in prevalence rates in different countries, regions of countries and in different ethnic groups. This can be explained in part by differences in definitions, measurement techniques, availability, price, social acceptability, seizure and arrest policies, and in patterns and modes of use. During pregnancy, up to 15% of women may be using alcohol and about 5% may be using illicit drugs. The proportion of women using substances is less at term than in the early stages of pregnancy. Despite this, substance use rises sharply in the first 6 months postpartum. Detection of substance use in obstetric units is low but perinatal substance misuse intervention reduces adverse neonatal outcomes. On the basis of the relatively high rate of substance use disorders during pregnancy, effective screening and intervention strategies should be implemented.