Adjuvants and antigen-delivery systems are essential in inducing and modifying immune responses, and despite the variety of materials available for such use, mechanisms by which they support immunity appear to be little known. A common denominator for most antigen-delivery systems is their particulate nature. Together with a certain depot effect, it is the particulate nature that primarily decides whether the antigen-delivery system will be successful in inducing an immune response. If this first requirement is fulfilled, the chemical composition of the vaccine decides which type of immune response will develop, e.g. which isotype of antibodies the B cells will produce, and which cytokines the T cells will secrete, and can be controlled by combining the antigen with immunomodulatory or co-stimulatory molecules. It is our goal to provide an overview of the cellular and molecular factors involved in the induction of immunity and how such factors may influence the potency of an adjuvant or a vaccine. Such factors should then be implemented in the design of new vaccines or in tuning the properties of existing vaccines in order to reach the properties that are necessary for successful vaccination.