The invention relates to an infectious arenavirus particle that is engineered to contain a genome with the ability to amplify and express its genetic information in infected cells but unable to produce further infectious progeny particles in normal, not genetically engineered cells. One or more of the four arenavirus open reading frames glycoprotein (GP), nucleoprotein (NP), matrix protein Z and RNA-dependent RNA polymerase L are removed or mutated to prevent replication in normal cells but still allowing gene expression in arenavirus vector-infected cells, and foreign genes coding for an antigen or other protein of interest or nucleic acids modulating host gene expression are expressed under control of the arenavirus promoters, internal ribosome entry sites or under control of regulatory elements that can be read by the viral RNA-dependent RNA polymerase, cellular RNA polymerase I, RNA polymerase II or RNA polymerase III. The modified arenaviruses are useful as vaccines and therapeutic agents for a variety of diseases.