I have attempted to illustrate the many different properties of retroviruses and their presence in a wide variety of animal species including humans. Since the turn of this century, progress in the field of retrovirology has been noteworthy and many new and important scientific observations have been made (Table 7). Along the way, certain dogmas were replaced with new tenets. The recent recognition of retroviruses associated with human cancer and immunodeficiency places them into consideration as potential agents responsible for other human diseases such as autoimmunity and multiple sclerosis. Not only can retroviruses be oncogenic or cytopathic agents but they can also exist highly conserved as endogenous genes in the chromosomal DNA of many different species and not cause disease. In fact, this latter group appears to be predominant, suggesting their role in normal developmental processes and as progenitors of the pathogenic types. The virus-like genomes recognized in Drosophila and other lower animal species could be examples of this fact and may represent important biological entities throughout nature. The genetic material of retroviruses resembles transposons and may reflect the ability of these viruses to be passed within the host and to affect the evolutionary pathway. They could, as transposable elements, be transmitted as well to many different animal species. By their ability to move within the genetic machinery of the cell, these viruses could influence development in animals through promotion, enhancement, or suppression of specific cellular genes. This idea has been proposed for the noninfectious type A particles that have been observed to show these effects in cultured cells. One important observation is that the effect of retroviruses on cells has a varied pattern which may be emphasized by one group (e.g., vacuolization by foamy virus) or shared by other groups (e.g.., syncytial cell formation by type C and type D oncovirinae, spumavirinae, and lentivirinae) (Table 6). Moreover, the heterogeneity of the lentiviruses and the transduction of normal cellular genes by many of the oncogenic viruses indicate the changes that can occur as retroviruses infect and replicate within the cell. The overview is very informative. Virus-cell interaction can lead to biological expressions that depend on the phenotype of the cell and the viral genetic structure. Throughout its existence in nature the retrovirus has been evolving, conserving certain features while developing new ones with different properties; it clearly emerges as a multifaceted agent.