Diabetes and obesity are frequently referred to as the “twin epidemics” of the modern world. The prevalence of both conditions is increasing worldwide, across high-, middle-, and lower-income countries. 1 Concurrently, the trend toward women having children at later ages, particularly in many developed countries, has led to an increasing prevalence of gestational and preexisting diabetes in pregnancy. 2 Gestational diabetes mellitus (GDM) and maternal obesity are both associated with a similar spectrum of risk for a broad range of pregnancy complications. 3 This chapter aims to consider these risks together, analyzing their contribution to overall pregnancy outcomes from several perspectives: (1) underlying pathophysiology (common and discrete mechanisms), (2) combined clinical effects of obesity and GDM, (3) the current evidence base for various modalities of treatment with emphasis on the findings of randomized controlled trials, (4) population risks including the potential for prevention, and (5) current evidence gaps, evidence–practice gaps, and areas for future research. While recognizing that intergenerational effects are potentially of great importance, for reasons of space we are unable to consider these in detail in the current review.