As an alternative to DNA-based vaccines, messenger RNA (mRNA)-based vaccines present additional safety features: no persistence, no integration in the genome, no induction of autoantibodies. Moreover, mRNA which are generated by in vitro transcription, are easy to produce in large amounts and very high purity. This feature facilitates the good manufacturing practices process and guaranties batch-to-batch reproducibility. Vaccination can be achieved by several delivery methods including direct injection of naked mRNA, injection of mRNA encapsulated in liposomes Gene Gun delivery of mRNA loaded on gold beads or in vitro transfection of the mRNA in cells followed by re-injection of the cells into the patients. Two of these technologies are being evaluated in human clinical trials: (1) in vitro mRNA-transfection of dendritic cells to be adoptively transferred and (2) direct injection of globin-stabilized mRNA. This chapter describes the production of mRNA and the preparation of the two types of mRNA-based vaccines tested in humans.