Maternal diabetes in pregnancy has short-term as well as long-term implications for the offspring. Observational studies in humans and animals as well as interventional studies in animals have found associations between fetal exposure to intrauterine hyperglycemia and later development of obesity, type 2 diabetes, and cardiovascular risk factors. The evolving field of epigenetics may open the black box of unanswered questions regarding the development of phenotype as a result of the complex interplay between genetic predisposition and fetal exposure to intrauterine hyperglycemia. The topic of this chapter is to give an overview of the long-term effects in the offspring of women with either pregestational or gestational diabetes mellitus in pregnancy. The chapter discusses metabolic and cognitive consequences and presents results from research within epigenetics and stem cell, aiming to explore pathogenic mechanisms leading from fetal exposure to later impaired health. The chapter primarily focuses on human studies.