Microorganisms are ubiquitously distributed in all types of lakes of varying trophic state. They play an important role in aquatic ecosystems, performing the tasks of decomposing organic matter, cycling nutrients and forming the base of microbial food webs. Bacterial abundance is generally lower in ultraoligotrophic Lake Superior than in other great lakes, ranging from 0.06 to 0.07 × 10 6 cells l -1 in offshore areas to 1.9 × 10 6 cells l -1 at nearshore stations along the north shore of this lake. Bacterioplankton cells are usually less abundant than in Lake Michigan and rarer than in more eutrophic Lake Erie. Heterotrophic bacterial production ranges from 0.02 to 0.66 mg C m -3 h -1 in the western arm of Lake Superior, which is generally lower than in the other Laurentian Great Lakes. There do not appear to be consistent gradients of bacterial abundance and production from nearshore to offshore areas and there are few studies about microbial populations and the diversity of microbial communities in Lake Superior. Yet, a better understanding of Lake Superior's smallest inhabitants - the prokaryotes- would improve our knowledge about how this lake functions and help us to better understand and possibly resolve new issues that are emerging. This paper reviews the current status of microbiological research in Lake Superior, emphasizing recent work on picoplanktonic Bacteria and Archaea, and attempts to place these findings and current ecological issues within a historical framework.