Approximately 45% of the human genome is occupied by transposable elements. Transposable elements have had an important impact on the structure and function of human genomes. Some of these elements are still capable of transposing, and their movements often cause diseases. Long interspersed nuclear element 1 (LINE-1) is the only active autonomous transposon in humans, and it mediates the mobilization of nonautonomous elements. Recently, great progress has been made in understanding the biology of LINE-1. Here, we briefly review the structure of LINE-1, the mechanism of its mobilization, and its impact on human genome and human health. We also highlight the mechanisms that host cell has evolved to control LINE-1 replication. Given the similarity of the life cycle between LINE-1 and retroviruses, the LINE-1 research may also shed new light on the biology of retroviruses.