Subunit vaccine formulations were prepared in the near past by purifying antigenic components known to activate protective immune responses directly from the pathogen. Nowadays, thanks to the development of high performance gene engineering and biochemical procedures, subunit vaccines commence to be formulated with recombinant versions of protective antigens. These molecules can be synthesized using heterologous hosts such as bacteria, yeast, insect and mammalian cells. To this aim, the alternative use of plants is growing out of advances in methods for foreign gene expression. Plants represent an opportunity in the field of vaccine technology in that this expression system ensures rapidity, low costs, easy scaling up and intrinsic bio-safety of the final product. Nonetheless, the exploitation of plants only as mere biofactories of antigens thwarts many of their potentialities. After a short introduction designed to initiate the reader to the most commonly used methods to engineer plant biofactories, the chapter will focus on how plants can be used by themselves as vaccines or as source of recombinant antigens with immunologic added value. © 2012 Springer Science+Business Media Dordrecht. All rights are reserved.