Some aspects of inorganic nitrogen conversion by microorganisms like N2O emission and hydroxylamine metabolism studied by Beijerinck and Kluyver, founders of the Delft School of Microbiology, are still actual today. In the Kluyver Laboratory for Biotechnology, microbial conversion of nitrogen compounds is still a central research theme. In recent years a range of new microbial processes and process technological applications have been studied. This paper gives a review of these developments including, aerobic denitrification, anaerobic ammonium oxidation, heterotrophic nitrification, and formation of intermediates (NO2-, NO, N2O), as well as the way these processes are controlled at the genetic and enzyme level.