Over the past decade therapeutic interventions during pregnancy, including the use of drugs, has increased; for example, the prescription use of opioids and nonopioid analgesics for pain management during pregnancy has increased by about 40%. Additionally, women also self-medicate during pregnancy with opioids, and are treated with methadone and buprenorphine which are used as substitution therapy for substance dependence during pregnancy. A nationally representative survey noted that ~74% of women of reproductive age reported substance use during the previous year, decreasing to ~63% amongst those who were pregnant. Substance dependence was reported by ~20% of women who were not pregnant, and ~15% of those who were pregnant. Understanding the consequences of opioid treatment or dependence during pregnancy requires an investigation of the placental disposition (metabolism and transfer), as well as of the impact of the drugs on the placental function. This review summarizes the research on the placental disposition of opioids and their impact on the placental function.