The alphaviruses Semliki Forest virus (SFV) and Sindbis virus have recently been developed as prototype anti-cancer agents. These are RNA-containing enveloped viruses that code for only 9 proteins of unique sequence. The standard recombinant SFV vector system consists of suicide particles containing recombinant RNA. In addition, alphavirus vectors capable of limited multiplication in the host are also being developed. Several strategies are being adopted to construct prototype SFV vectors for cancer treatment. These include: 1) construction of both prophylactic and therapeutic vaccines to stimulate immunity to tumor-associated antigens, 2) use of apoptosis induction to destroy tumor cells, which includes both the use of the inherent apoptosis-inducing ability of the vector and the action of pro-apoptotic genes cloned into the vector, and 3) expression of cytokines and other immunoregulatory proteins by the vector that enhance anti-tumor immune responses and/or inhibit tumor cell growth. This includes the use of cytokines such as IL-12 that target angiogenesis. Sindbis virus appears to have a natural tropism for tumor cells that may allow targeting both of the wild-type virus and the vector. This approach may also be useful for targeting metastases. For SFV, neurovirulence and/or neurotropism, as well as other tissue damage, may preclude the use of unmodified replication competent wild-type virus in tumor treatment. However, it may be possible to use such a virus in animals that have been vaccinated, using a vector-derived vaccine.