Professor Kjelleberg has made major contributions to microbial ecology. His studies on bacterial adaptive responses and biofilm biology have received strong international recognition and have illuminated the predominant modes of bacterial life in the environment. Moreover, his findings have laid the foundation for interdisciplinary research programs on interkingdom signalling and chemically mediated interactions between bacteria and higher organisms in marine systems, His discoveries of naturally derived antagonists of bacterial cell-cell signalling systems, the means by which bacterial biofilms and protozoans interact, and the mechanisms of biofilm differentiation and dispersal have also greatly contributed to ecological theory and environmental biotechnology.