In this chapter, the authors review how the adaptive immune response protects against the attack of pathogens, describing the major types of lymphocytes, B and T cells, and the different forms of immune protection they provide. They discuss how best to deliver vaccines using adjuvants and delivery platforms, how to differentiate vaccinated from infected animals, how to assess protection after vaccination, and the features of active, passive, and herd immunity. It is important for vaccines to induce the type of immunity able to neutralize the pathogen: antibody and/or cell-mediated immunity. Emergency vaccination using DIVA vaccines could be an important control tool for disease outbreaks in densely populated livestock areas. DIVA vaccination might also limit the number of culled animals in the process of disease eradication, thereby enhancing public acceptance of disease control measures and limiting economic damage. Therefore, the DIVA concept is recommended for inclusion in any new vaccine development.