Insulin therapy is essential for optimal glycemic control during pregnancy in women with type 1 diabetes and is frequently required to optimize control in women with type 2 diabetes. Less commonly, women with gestational diabetes mellitus (GDM) require insulin for glycemic control. However, because of its greater prevalence, GDM is the most common reason for insulin use in pregnancy. The most frequently used insulin regimen in pregnancy is a basal/bolus combination of long- and short-acting insulin preparations. There is no evidence base to support one treatment regimen over another. Therapy should be individualized and based on local expertise.