Recombination in RNA viruses is a general phenomenon, and is considered to play a major role as a driving force in virus variability and thus in virus evolution.  An ever increasing number of RNA viruses have been shown to undergo RNA recombination, whether under natural or experimental conditions.  More than 30 years after its discovery, the mechanisms of viral RNA recombination are only now beginning to be elucidated.  Recent reports strongly suggest that RNA recombination is linked to virus replication, and that it occurs by a copy-choice mechanism.  The detection of recombination between host transcripts and infecting viral RNA genomes has given rise to new concerns about the release of virus-resistant transgenic crops, since recombination could generate viruses with properties different from the infecting strain.  We will describe here current knowledge of recombination in RNA viruses, and then will present results showing recombination in transgenic virus-infected plants, and finally will discuss to what extent this may lead to increased variability in plant RNA viruses.