Diabetes mellitus (DM) is the most common medical complication of pregnancy with approximately 85% of cases representing gestational diabetes mellitus (GDM). The rising prevalence of DM among women of childbearing age parallels the increasing incidence of obesity. This chapter covers the physiology of diabetes during pregnancy, maternal risks with diabetes, fetal risks with maternal diabetes, preconception counseling for pregestational diabetes, gestational diabetes, and management clinical pearls for diabetes mellitus in pregnancy. Gestational diabetes is the term used to describe when diabetes is first diagnosed during pregnancy. Most use the term gestational diabetes to describe patients diagnosed during the second half of pregnancy and the terms overt diabetes or diabetes mellitus in pregnancy to identify patients diagnosed by the standard nonpregnant criteria early in pregnancy when the effects of pregnancy induced insulin resistance are minimal.