Originally developed as a new category of retroviral vectors that are capable of transducing non-dividing cells, vectors based on lentiviruses have been shown to incorporate a number of additional features that are of potential value for clinical gene therapy. These include the utilisation of biological properties of the lentiviral accessory proteins Tat and Rev, which allow conditional mRNA expression and mediate a “stabilisation” of the genomic vector RNA in packaging cells; the integration pattern, which, when compared to gammaretroviral vectors, is less likely to affect promoter-proximal windows or regulatory regions located in DNAse1 hypersensitive sites of cellular genes; and a relatively robust gene expression even in cells that are at relatively high risk of epigenetic transgene silencing. Here, we discuss the mechanisms underlying these potential advantages and their importance for the development of clinical grade gene vectors. We conclude with an overview of clinical trials in which lentiviral vectors have been or are currently being used to counteract advanced forms of HIV infection, treat inherited disorders affecting hematopoietic cells, or transduce neuronal cells of the central nervous system for the treatment of Parkinson disease. As information on most clinical trials is not yet available in the form of peer-reviewed papers, this list may be incomplete. Some additional applications that are expected to lead to the initiation of clinical trials in the near future are also discussed.