Modulating immune responses to pathogen invasion and even tumors is a major goal in immunotherapy. T cells play a central role in these responses. Progress towards that goal is accomplished by stimulating the antigen-specific T cell immune response in vivo through active immunization, or by re-transfer of large numbers of T cells expanded outside the body in a process called adoptive immunotherapy. In both vaccination and adoptive cellular therapy, there is a critical need for a reliable and effective antigen-presentation strategy that stimulates T cells in a specific and efficient manner. Biodegradable nanoparticles can be engineered with bacterial lipopolysaccharides coating thus priming dendritic cells for improved immunization. Alternatively, micron-sized particles can be made to approximate the natural ability of dendritic cells in stimulating T cells by surface modification with the appropriate T cell antigens. Here we show how both of these approaches can be employed to produce safe and effective vaccine and cellular therapeutics.