Haustoria are morphological features of an extremely successful class of plant parasites, the obligate biotrophs. The broad phylogenetic spectrum of organisms producing haustoria suggests that these structures have arisen many times in the course of evolution and represent specific adaptations of these organisms to the close interaction with their respective host plants. This close interaction and the fact that these structures cannot be produced in vitro have hampered an analysis of the roles of haustoria in biotrophy for many decades. Only recently has it become possible to analyse haustorial function at a molecular level. A picture is beginning to emerge indicating that haustoria do not only serve in nutrient uptake – a task postulated for these elements ever since their discovery. Moreover, they seem to perform enormous biosynthetic duties. They also seem to be engaged in the suppression of host defense responses and in redirecting or reprogramming the host's metabolic flow. This review intends to summarize current knowledge about the structure and function especially of rust haustoria.