In this review, we discuss the impact of migration on the incidence and prevalence of obesity and type 2 diabetes mellitus (T2DM) in different ethnic groups and populations. We also analyze the determinants of such phenomena in view of the global increase in the migration and escalating prevalence of obesity and T2DM. The risk escalation of the obesity and T2DM followed a gradient, as migrants (Blacks, Hispanics, Chinese, South Asians, etc.) became more affluent and urbanized, indicating an important role of environmental factors. A stepwise increase in the prevalence of obesity in Blacks along the path of migration (5% in Nigeria, 23% in Jamaica, and 39% in the United States) is a classic example. Furthermore, South Asian migrants, who are particularly predisposed to develop insulin resistance and T2DM, showed nearly four times prevalence rates of T2DM than rural sedentee populations. Similar observations were also reported in intracountry migrants and resettled indigenous populations. The determinants were found to include nutrition transition, physical inactivity, gene-environment interaction, stress, and other factors such as ethnic susceptibility. However, certain contradictory trends were also seen in some migrant communities and have been explained by various phenomena such as healthy migrant effect, "salmon bias", and adherence to traditional diets. A review of the evidence suggests a critical role of environmental factors in conferring an increased risk of obesity and T2DM. The important contributory factors to this phenomenon were urbanization, mechanization, and changes in nutrition and lifestyle behaviors, but the role of stress and as yet unknown factors remain to be determined.