Diseases caused by Neisseria meningitidis and Neisseria gonorrhoeae, are a significant health problem worldwide, the control of which are largely dependent on the availability and widespread use of comprehensive meningococcal and gonococcal vaccines. Unfortunately, the development of Neisserial vaccines has been challenging due to the nature of these organisms, in particular the heterogeneity, variability and/or poor immunogenicity of their outer surface components. As strictly human pathogens, they are highly adapted to the host environment but have evolved several mechanisms to remain adaptable to changing microenvironments and avoid elimination by the host immune system. Currently, serogroup A, C, Y and W-135 meningococcal infections can be prevented by vaccines. However there is no comprehensive serogroup B vaccine, and the prospect of developing a gonococcal vaccine is remote.