We have described the properties of glucose transporters expressed in several mammalian tissues and have summarized some of the adaptations that take place involving these molecules in various normal and abnormal states. With the exception of a few cell types, such as adipocytes and skeletal muscle, glucose transport is not a rate-limiting step in cellular glucose metabolism, and other substrates may be equally important for cellular metabolism. Nevertheless, an understanding of the mechanisms behind the regulation of glucose transport in individual tissues may facilitate an understanding of in vivo glucose utilization and clearance processes as they relate to normal and disease states. Although adult studies provide an impetus toward a mechanistic approach in preventing and treating various disease states involving derangements in glucose homeostasis, there remains a need for similar studies in the fetus and newborn. These developmental studies should help unravel the fetal/neonatal responses to normal and abnormal hormonal and substrate milieu.