Bacterial kidney disease (BKD) is one of the most important diseases of wild and cultured salmonid fish, and has been reported from many different countries. Despite considerable effort, many pieces are still missing from the "jigsaw puzzle" which represents our knowledge of the disease, and its etiological agent Renibacterium salmoninarum. The purpose of this review is to consider current knowledge about this bacterial pathogen and the pathogenesis of BKD. It is our intention to construct a picture of the possible ways in which pathogen and host interact, in particular by high-lighting the areas where our understanding is poor and indicating how recent advances in methodology offer the prospect of improvement. The exact status of R. salmoninarum as an obligate fish pathogen, and the implications for transmission, entry, colonization, and disease progression are considered. We review the considerable progress that has been made recently through work on the molecular determinants of pathogenicity, particularly attachment, cellular invasion mechanisms, intracellular survival of the bacterium, and its interactions with the host defense mechanisms and immune system. Finally, we consider ways in which the future control of BKD through improved diagnosis, chemotherapy, and vaccination may be realized through an integrated approach to the study of BKD.