Gestational diabetes mellitus (GDM) is the most common metabolic disease of pregnancy and is associated with several perinatal complications. GDM is defined as diabetes diagnosed in the second or third trimester of pregnancy that was not clearly overt diabetes prior to gestation. In Europe, in 2016, the prevalence of GDM was estimated to be 5.4% (3.8-7.8). It varied depending on maternal age, year of data collection, country, area of Europe, week of gestation at testing, and diagnostic criteria. The Hellenic Endocrine Society and the Hellenic Society of Maternal-Fetal Medicine commissioned an expert group to construct national guidelines on "Diabetes mellitus and pregnancy: Gestational diabetes mellitus." Following a search for the best available evidence and critical appraisal of the results, the writing group generated a series of consensus recommendations regarding screening tests for the general population, monitoring and management, fetal monitoring, management of preterm labor, planning of labor and delivery, puerperium and breastfeeding, and long-term follow-up of GDM.