The 5th Pathogenic Neisseriae Conference was held in Noordwijkerhout, The Netherlands, on 15-18 September 1986. It followed San Francisco - USA (1978), Hemavan - Sweden (1980), Montreal - Canada (1982), Asilomar - USA (1984). These conferences will continue to be held in the foreseeing future. The reason for this is because studying pathogenic Neisseriae enables research workers to look at many aspects of bacterial infectious diseases. These aspects include: - Epidemiology and vaccination (section 1) - Genetic aspects of antigenic diversity (section 2) - Outer membrane proteins and IgA-protease (section 3) - Lipopolysaccharides, H8 antigen and peptidoglycan (section ~) - The application of all this knowledge to study pathogenesis (section 5) N. gonorrhoeae has become an important model organism to study mucosal bacterial infectious diseases. Basic knowledge about the bacterial surface, gene tics and physiology is linked to pathogenic mechanisms such as adhesion to and invasion of epithelial cells, interaction with complement and phagocytic cells and mucosal immune mechanisms. This will continue to be an important research field in the near future. N. meningitidis has become an important model organism to study the epide miology of potentially epidemic bacterial diseases and against which new type of vaccines are being developed such as outer membrane protein vaccines. The above mentioned topics are covered by papers presented at The 5th Path ogenic Neisseriae Conference, 1986. I would like to thank Marijke van de Nadort without whom it would have been much harder or even impossible to organize such a meeting.