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Like all retroviruses, prototypic simple (MuLV) and complex (HIV-1) viruses are enveloped positive strand RNA viruses which replicate through a DNA copy integrated in the host cell genome. The viral genome is composed of a dimer of two identical RNA molecules and their encapsidation in the viral particle results from a selective uptake of the viral genome-length RNA among abundant cellular RNAs. Because RNA encapsidation also impacts on several essential functions of viral replication such as RNA dimerization, translation and recombination events, we can hope that knowledge of the RNA packaging mechanism may lead to the development of new antiviral drugs (1) and to the improvement of successful and safe retroviral vectors for use in gene therapy strategies (2). This review focuses on recent research directed at elucidating the mechanism of RNA encapsidation. Encapsidation requires the presence of a cis-acting signal (Psi) in the RNA sequence which must be selectively recognized by the viral assembling Gag polyprotein. Recently, the existence of a novel packageable subgenomic RNA has been shown in MuLV although it was commonly thought that only unspliced RNA is efficiently incorporated into nascent virions. This, together with recent data on intracellular viral assembly, leads to a revisited view of the viral RNA encapsidation pathway.