Childhood obesity has reached pandemic proportions, and youth-onset type 2 diabetes is following suit. This review summarises the literature on the influence of developmental overnutrition, resulting from maternal diabetes, obesity, maternal dietary intake during pregnancy, excess gestational weight gain, and infant feeding practices, on the aetiology of obesity and type 2 diabetes risk during childhood and adolescence. Key goals of this review are: (1) to summarise evidence to date on consequences of developmental overnutrition; (2) describe shared and distinct biological pathways that may link developmental overnutrition to childhood obesity and youth-onset type 2 diabetes; and (3) to translate current knowledge into clinical and public health strategies that not only target primary prevention in youth, but also encourage primordial prevention during the perinatal period, with the aim of breaking the intergenerational cycle of obesity and diabetes.