Buprenorphine and methadone are opioid-receptor agonists used as opioid substitution therapy during pregnancy to limit exposure of the fetus to cycles of opioid withdrawal and reduce the risk of infectious comorbidities of illicit opioid use. As part of a comprehensive care plan, such therapy may result in improved access to prenatal care, reduced illicit drug use, reduced exposure to infections associated with intravenous drug use, and improved maternal nutrition and infant birth weight. This article describes differences in patient selection between the two drugs, their relative safety during pregnancy, and changes in daily doses as a guide for prescribing clinicians.