Interferons specify a complex antiviral response that upon the detection of pathogens through various cellular pattern-recognition receptors (PRRs) lead to the induction of hundreds of genes named interferon-stimulated genes (ISGs). Several ISGs have been reported to restrict viral infection, however the antiviral role/s of many of them remains either unknown or poorly characterized. During my thesis I have focused on the characterization of ISG20 during the replication of two viruses, VSV and HIV-1. ISG20 had been previously identified as an antiviral 3’-5’ exonuclease and was thought to act by directly degrading viral genomes. However, the decrease in viral RNAs specified by ISG20 was controversial. To gather further insights into the mechanism with which ISG20 interfered with viral replication, I constructed several mutants of ISG20. The results we have obtained indicated that the antiviral activity of ISG20 does not solely rely on it's the ability of ISG20 to degrade RNA, as several mutants were identified that lost their antiviral properties despite a robust RNase capacity in vitro.We have found here that ISG20 could block viral replication through a block in translation. This block occurred both during viral infection as well as during the ectopic expression of reporter genes in ISG20-expressing cells. The results we have obtained indicate that ISG20 affects both cap- and IRES-mediated translation in a manner that is very likely independent from translation initiation.To substantiate the antiviral role of ISG20 during viral infection, knock-out isg20 -/- mice were generated and then analyzed for their ability to support VSV infection in vivo. The results obtained, clearly implicate ISG20 in the natural control of viral spread in vivo, strongly supporting our data ex vivo.Overall, the data obtained during my thesis indicate that ISG20 is an important antiviral factor and shed light on a novel mechanism of viral inhibition whereby ISG20 interferes with viral mRNA translation.