The RNA genomes of alphaviruses have been exploited to create highly efficient vectors for transient expression of foreign genes in mammalian cells and for use as vehicles for genetic vaccines. For Semliki Forest virus (SFV), a representative alphavirus, three strategies for in vitro and in vivo gene-delivery have been developed. One method relies on the packaging of recombinant vectors into suicidal viral particles and infection of target cells, whereas the other two methods are based on direct transfection of target cells, either by using naked DNA encoding the SFV replicon placed downstream of an RNA polymerase II dependent promoter, or by using in vitro transcribed RNA encoding the SFV replicon. All three approaches result in the delivery of a selfreplicating SFV vector into target cells, with expression of foreign genes being driven from a highly efficient viral subgenomic promoter.