The HIV-1 transactivator Tat protein plays a key role in AIDS pathogenesis. Besides the Tat role as activator of HIV-1 transcription, it exerts several important functions on infected and uninfected cells. In fact, HIV-1 Tat is released by infected cells and is taken up by neighboring cells. In this way it regulates expression of viral and cellular genes and it modulates several cellular pathways leading to HIV-1 infection spreading and immune dysregulation. So far, Tat protein and the cellular pathways targeted by Tat may represent potential targets for new anti-HIV therapeutic approaches and vaccine development against AIDS.