Nanotechnology provides an excellent platform for the development of a new generation of vaccines. These are based on purified subunit proteins or polysaccharides, recombinant proteins, synthetic peptides, or nucleic acids. These types of vaccines may be insufficiently immunogenic, thus requiring adjuvants that augment their immunogenicity. Nanoparticles (NPs) can act as adjuvants for vaccines, hence they are referred to as a nano-adjuvant (NA). NPs can either encapsulate or adsorb the vaccine antigen or DNA in an appropriate formulation, thus increasing stability, cellular uptake, and immunogenicity. In addition, the biodistribution and systemic release of a vaccine can also be controlled by different NA formulations. This review provides an overview of the classification of NAs and also addresses factors influencing the stability, release, and immunogenicity of the formulated vaccine. A basic understanding of these factors enables a more rational design of NA formulations. Applications of NAs and key challenges in their formulation development are also discussed.