First brought to scientific attention as infectious cancer-causing agents nearly 80 years ago, retroviruses are popular in contemporary biology for many reasons. (i) The virus life cycle includes several events--in particular, reverse transcription of the viral RNA genome into DNA, orderly integration of viral DNA into host chromosomes, and utilization of host mechanisms for gene expression in response to viral signals--which are broadly informative about eukaryotic cells and viruses. (ii) Retroviral oncogenesis usually depends on transduction or insertional activation of cellular genes, and isolation of those genes has provided the scientific community with many of the molecular components now implicated in the control of normal growth and in human cancer. (iii) Retroviruses include many important veterinary pathogens and two recently discovered human pathogens, the causative agents of the acquired immunodeficiency syndrome (AIDS) and adult T cell leukemia/lymphoma. (iv) Retroviruses are genetic vectors in nature and can be modified to serve as genetic vectors for both experimental and therapeutic purposes. (v) Insertion of retroviral DNA into host chromosomes can be used to mark cell lineages and to make developmental mutants. Progress in these and other areas of retrovirus-related biology has been enormous during the past two decades, but many practical and theoretical problems remain to be solved.