Gestational diabetes mellitus (GDM) is one of the most common complications of pregnancy and the prevalence of GDM is increasing worldwide. Short- and long-term complications of GDM on mothers and fetuses are well-recognized. These include more than seven-fold higher risk for type 2 diabetes mellitus (T2DM) later in life in women with GDM than those without. Evidence supports that GDM shares several risk factors with T2DM, including genetic risks. This chapter reviewed studies on candidate genes shared by T2DM and GDM published from 1990 to 2011. At least 20 susceptible genes of T2DM have been studied in women with GDM in various races. Results from current association studies on T2DM susceptible genes in GDM have shown significant heterogeneity There may be primary evidence that polymorphisms of susceptible genes of T2DM such as transcription factor 7-like 2 (TCF7L2) gene, potassium channel voltage-gate KQT-like subfamily member 1 (KCNQ1) gene, and cyclin-dependent kinase 5 regulatory subunit-associated protein 1-like 1 (CDKAL1) gene, may increase risk of GDM. Associations between GDM and many genetic variants have led to different findings across populations. Many genetic polymorphisms related to GDM were investigated in a single study or a single population. Replication studies to verify contributions of both common and rare genetic variants for GDM and T2DM in specific racial/ethnic groups are needed.