For well over 80 years, alum is the most widely used adjuvant.The use of alternative adjuvants has been explored, however, aluminum adjuvants will continue to be used for many years. This is due to their good track record of safety, low cost, and adjuvanticity with a variety of antigens. Surprisingly, itsmechanism of action remains largely unknown. In this book chapter we will describe the different alum formulations and our current understandingof its working mechanism, although alum’s final mode of action is not definite yet.Vaccinations have been given for well over a 100 years at the moment. The first reported vaccination was done by Edward Jenner in 1796 [1, 2]. He inoculated a young boy with cowpox virus and thereby rendered him resistant to a subsequent challenge with smallpox virus, an experiment that today would most certainly not be approved by regulatory agencies. Protection by vaccination can be achieved by giving inactivated microbes of virus particles, live attenuated virus, or subunit vaccine. However, subunit vaccination does not induce a strong immune response, which can be achieved by the administration of an adjuvant (Latin verb adjuvare means to help/aid). In immunology, an adjuvant is an agent that may stimulate the immune system and increase the response to a vaccine, without having any specific antigenic effect in it.