Gestational diabetes mellitus (GDM) is a pancreatic function insufficiency to overcome the insulin resistance associated with the pregnant state. The prevalence of GDM is about 2-5 % of normal pregnancies. To prevent the adverse outcome for the mother, the fetus, and neonatal period, detection of GDM is important. Screening methods vary by clinics. Screening can either be selective, based upon risk stratification or universal. Timely testing enables the obstetrician to assess glucose tolerance in the presence of the insulin-resistant state of pregnancy and permits treatment to begin before excessive fetal growth has occurred. After the diagnosis of GDM, close perinatal surveillance is necessary. The goal of treatment in GDM is to reduce fetal and maternal morbidity and mortality. The exact glucose values needed are still unknown. The decision whether and when to induce delivery depends on gestational age, fetal growth, maternal glucose levels, and Bishop score. Screening, follow-up, and treatment of GDM still have controversies.