Conserved in all primate lentivirus genomes, Nef promotes viral replication and infectivity, influences the trafficking of a large number of surface receptors and interferes with TCR signalling, consequently modulating T-cell activation. In vivo observations with Long Term Non-Progressors harbouring a Nef-defective HIV and vaccination studies with Nef-deleted SIV in Rhesus macaques have shown a prominent role for Nef in lentiviral pathogenesis. Here we review the functions of Nef involved in viral replication and infectivity and speculate on a possible role for Nef in HIV fitness.