Preeclampsia is diagnosed in women presenting with new onset hypertension accompanied by proteinuria or other signs of severe organ dysfunction in the second half of pregnancy. Preeclampsia risk is increased 2- to 4-fold among women with type 1 or type 2 diabetes. The limited number of pregnant women with preexisting diabetes and the difficulties associated with diagnosing preeclampsia in women with proteinuria prior to pregnancy are significant barriers to research in this high-risk population. Gestational diabetes mellitus (GDM) also increases preeclampsia risk, although it is unclear whether these two conditions share a common pathophysiological pathway. Nondiabetic women who have had preeclampsia are more likely to develop type 2 diabetes later in life. Among women with type 1 diabetes, a history of preeclampsia is associated with an increased risk of retinopathy and nephropathy. More research examining the pathophysiology, treatment, and the long-term health implications of preeclampsia among women with preexisting and gestational diabetes is needed.