The Semliki forest virus (SFV), a small envelopped virus of the Togaviridæ family, has allowed the development of a new expression system. Due to the increasing utilization of recombinant viral vectors as expression vectors, plasmids derived from this alphavirus replicon find gradually increasing utilizations. With its many advantages, such as a large host spectrum and easy utilization, the SFV expression system stands out from other viral expression systems. The SFV system has two efficient expression ways : the utilization of recombinant SFV RNA, and the production of recombinant SFV suicide particles. The power of these two complementary approaches confers to the SFV system a promising future as vaccinal vector, wich represents, for the moment, the majority of its applications. In addition to the vaccine domain, the SFV system applications are diversifying, and the great efficacity of this system associated to a high biologic safety, open it the way to gene therapy.