Little has been known about the components of the immune system that are effective in the protection of a pig against PRRSV infection. Although antibodies were initially perceived as a deleterious, ineffective component of the PRRSV-specific immune response, neutralizing antibodies (NA) are now considered to be an important correlate of protective immunity against PRRSV. This paper reviews the current knowledge on arterivirus-specific NA, the role that NA have in protection against infection with PRRSV, as well as the viral molecular structures that are responsible for the production of this type of antibodies by the pig. This information should prove central to the design of new generation vaccines against PRRSV.