In general terms, the replication cycle of lentiviruses, including HIV-1, closely resembles that of other retroviruses. There are, however, a number of unique aspects of HIV replication; for example, the HIVs and SIVs target receptors and coreceptors distinct from those used by other retroviruses. Lentiviruses encode a number of regulatory and accessory proteins not encoded by the genomes of the prototypical "simple" retroviruses. Of particular interest from the gene therapy perspective, lentiviruses possess the ability to productively infect some types of non-dividing cells. This chapter, while reiterating certain points discussed in Chapter 1, will attempt to focus on issues unique to HIV-1 replication. The HIV-1 genome encodes the major structural and non-structural proteins common to all replication-competent retroviruses (Fig. 1, and Chapter 1). From the 5'- to 3'-ends of the genome are found the gag (for group-specific antigen), pol (for polymerase), and env (for envelope glycoprotein) genes. The gag gene encodes a polyprotein precursor whose name, Pr55Gag, is based on its molecular weight. Pr55Gag is cleaved by the viral protease (PR) to the mature Gag proteins matrix (also known as MA or p17), capsid (CA or p24), nucleocapsid (NC or p7), and p6. Two spacer peptides, p2 and p1, are also generated upon Pr55Gag processing. The pol-encoded enzymes are initially synthesized as part of a large polyprotein precursor, Pr160GagPol, whose synthesis results from a rare frameshifting event during Pr55Gag translation. The individual pol-encoded enzymes, PR, reverse transcriptase (RT), and integrase (IN), are cleaved from Pr160GagPol by the viral PR. The envelope (Env) glycoproteins are also synthesized as a polyprotein precursor (Fig. 1). Unlike the Gag and Pol precursors, which are cleaved by the viral PR, the Env precursor, known as gp160, is processed by a cellular protease during Env trafficking to the cell surface, gp160 processing results in the generation of the surface (SU) Env glycoprotein gp120 and the transmembrane (TM) glycoprotein gp41. gp120 contains the determinants that interact with receptor and coreceptor, while gp41 not only anchors the gp120/gp41 complex in the membrane (Fig. 2), but also contains domains that are critical for catalyzing the membrane fusion reaction between viral and host lipid bilayers during virus entry. Comparison of env sequences from a large number of virus isolates revealed that gp120 is organized into five conserved regions (C1-C5) and five highly variable domains (V1-V5). The variable regions tend to be located in disulfide-linked loops. gp41 is composed of three major domains: the ectodomain (which contains determinants essential for membrane fusion), the transmembrane anchor sequence, and the cytoplasmic tail. In addition to the gag, pol, and env genes, HIV-1 also encodes a number of regulatory and accessory proteins. Tat is critical for transcription from the HIV-1 LTR and Rev plays a major [figure: see text] role in the transport of viral RNAs from the nucleus to the cytoplasm. Vpu, Vif, Vpr and Nef have been termed "accessory" or "auxiliary" proteins to reflect the fact that they are not uniformly required for virus replication. The functions of these very interesting proteins will be discussed in more detail at the end of this chapter. HIV replication proceeds in a series of events that can be divided into two overall phases: "early" and "late" (Fig. 3). Although some events occur in a concerted or simultaneous fashion, the replication cycle can be viewed most simply as proceeding in an ordered, step-wise manner. In this chapter, each step in virus replication will be considered; additional information can be obtained from the more detailed reviews and primary references that are cited.