There is overwhelming evidence for an involvement of reactive oxygen species (ROS) in the pathogenesis of diabetes-associated vascular complications. However, neither the exact source of the ROS initiating cascades leading to cell dysfunction in diabetes nor their chemical nature is fully understood. Furthermore, despite our knowledge of the crucial role of ROS in diabetes, little is known about the actual targets and the molecular consequences of the interaction of ROS with cellular signalling pathways. Therefore, we aim to provide an overview of ROS (i.e. O2(*-), NO*, ONOO- and H2O2) and their vascular sources in diabetes and to summarise recent knowledge on the mechanisms underlying increased ROS production within the vascular wall. In addition, possible targets of diabetes and ROS within the vasculature are discussed. These include, the effects of ROS on small guanine nucleotide binding proteins, the cytoskeleton, protein kinases (e.g tyrosine kinases), metalloproteinases, ion homeostasis and transcriptional regulation. Such analysis makes it clear that the generation of ROS could affect a large number of various signalling pathways and proteins. Thus, a better knowledge of the functional diversity and pathological consequences of each individual pathway activated by ROS id essential to understand the mechanisms of diabetes-associated vascular complications.