Hyperglycemia is increasingly regarded as the cause of the diabetic complications, in particular via the ability of glucose to glycate proteins and generate Maillard browning products which cross-link proteins and render them brown and fluorescent in vitro. Similar changes occur in vivo to long-lived proteins in diabetes mellitus as well as in ageing. The evidence supporting this route of glucose toxicity is discussed in the context of the ability of glucose to oxidize in vitro (catalyzed by trace amounts of transition metal) generating hydrogen peroxide, highly reactive oxidants, and protein-reactive ketoaldehyde compounds. It is suggested that protein browning in vivo may not result from the reactions of glucose with protein but from the transition metal-catalyzed reactions of other small autoxidisable substrates, such as ascorbate, with protein. Overall, studies of glycation and protein browning suggest a critical role for oxidative processes perhaps involving decompartmentalized transition metals and a variety of low molecular weight reducing agents in diabetes mellitus and ageing.