Vaccines represent one of the most successful strategies in medical science. From the mechanistic perspective, vaccination works by manipulating the immune response through selecting, activating and expanding the memory of B and T cells. To determine the magnitude and quality of immune response, suitable vaccine adjuvants are required; therefore, much effort is going into finding new, effective and non-toxic adjuvant formulations focussed on the activation of key immune targets for inducing a long-term, potent and safe immune response. Significant research is being done in this area, to develop new classes of vaccines for use not only against infectious diseases, but also in the treatment of autoimmune disorders, allergies, chronic inflammatory diseases and cancer. Here the authors review and classify some of the main vaccine adjuvants on the basis of their immunomodulating properties on the immune system.