Retroviral DNA integration into the host cell genome is an essential feature of the retroviral life cycle. The ability to integrate their DNA into the DNA of infected cells also makes retroviruses attractive vectors for delivery of therapeutic genes into the genome of cells carrying adverse mutations in their cellular DNA. Sequencing of the entire human genome has enabled identification of integration site preferences of both replication-competent retroviruses and retroviral vectors. These results, together with the unfortunate outcome of a gene therapy trial, in which integration of a retroviral vector in the vicinity of a protooncogene was associated with the development of leukemia, have stimulated efforts to elucidate the molecular mechanism underlying integration site selection by retroviral vectors, as well as the development of methods to direct integration to specific DNA sequences and chromosomal regions. This review outlines our current knowledge of the mechanism of integration site selection by retroviruses in vitro, in cultured cells, and in vivo; the outcome of several of the more recent gene therapy trials, which employed these vectors; and the efforts of several laboratories to develop vectors that integrate at predetermined sites in the human genome.