The human immunodeficiency viruses (HIVs), in particular HIV-1, are the causative agent responsible for the current worldwide epidemic of acquired immunodeficiency syndrome (AIDS). A major effort has thus been underway over the past two decades to understand and control this pathogen. During this time, an enormous knowledge base has accumulated regarding the role of viral factors in the HIV-1 life cycle, and the interaction of HIV-1 with the host cell is becoming increasingly understood. Certain features of HIV, for example its ability to infect non-dividing cells, are being exploited in the development of novel gene therapy vehicles. This volume provides an overview of the current information regarding the HIV replication cycle and will serve as an introduction to subsequent chapters that address specific aspects of lentiviral-based gene therapy.