The Tat protein of HIV-1 is a powerful transactivator of gene expression. By interacting with a structured RNA sequence at the 5' end of the viral mRNA, it promotes the remodeling of chromatin and the recruitment of processive RNA polymerase complexes at the viral promoter. In addition to these transcriptional functions, a short amino acid motif, highly enriched in basic amino acids, promotes the export of the protein from the expressing cells. Once in the extracellular environment, the same basic domain of Tat binds to cell surface heparan sulfate proteoglycans; through this interaction, the protein is internalized by a variety of different cell types. Cellular internalization of Tat and Tat fusion proteins requires the integrity of cell membrane lipid rafts and mainly occurs through caveolar endocytosis. The Tat basic domain, when attached to large protein cargos, also mediates their efficient cellular internalization and can be thus utilized for transcellular protein transduction. This property has already been successfully exploited for the delivery of heterologous proteins, nanoparticles, liposomes, phage and viral vectors, and plasmid DNA. The biological significance of intercellular Tat trafficking in the context of viral infection still remains elusive.