HIV-1 replication is a dynamic process influenced by a combination of viral and host factors. The HIV-1 matrix protein p17 is a structural protein critically involved in most stages of the life cycle of the retrovirus. It participates in the early stages of virus replication as well as in RNA targeting to the plasma membrane, incorporation of the envelope into virions and particle assembly. Besides its well established functions, p17 acts as a viral cytokine that works on preactivated--but not on resting--human T cells promoting proliferation, proinflammatory cytokines release and HIV-1 replication after binding to a cellular receptor (p17R). Thus, p17 might play a key role in the complex network of host- and virus-derived stimulatory factors contributing to create a favourable environment for HIV-1 infection and replication.