American foulbrood, a globally spread bacterial disease of honeybee brood, is one of the most deleterious bee diseases. Its etiological agent is the gram-positive, spore-forming bacterium Paenibacillus larvae. This review will focus on recent achievements in the study of Paenibacillus larvae brought about by molecular methods introduced into the field over the last fifteen years. One topic will be the classification of the etiological agent which changed several times since the first description in 1906 and was most recently modified again. The relevance of this reclassification for laboratory diagnosis of American foulbrood will be covered. The analysis of differences in virulence of Paenibacillus larvae and the implications of these differences for transmission of the pathogen and clinical diagnosis of American foulbrood will be another thematic complex.