Retroviridae comprise viruses with diploid RNA genome, which are replicatedby RNA-dependent DNA polymerase, i.e. reverse transcriptase. This enzyme itself is a valuable tool of molecular biology. However, retroviruses are in the forefront of scientific interest mainly due to the serious diseases they cause. In addition, elucidation of some aspects of the cell cycle andcell ular oncogens originated from the studies of retroviral oncogenesis. Another attractive area is the design of retroviral vectors for gene therapies. This review attempts to sum up the most recent general knowledge of the function of retroviral genes and their products in the virus life cycle.