Viruses are parasites and are considered to be simplest things that exhibit the properties of living systems. They can produce copies of themselves, although only within the host cells, and can change through mutations. Depending upon the host which a virus infects, these can be broadly placed in three categories : (i) animal viruses, (ii) plant viruses and (iii) bacterial viruses or bacteriophages. Simplest of these three classes are bacteriophages. Bacteriophages are also a class of viruses, which have been most extensively utilised for study of genetics of viruses. Contemporary with the discovery of sexuality in bacteria in 1946 by Lederberg and Tatum, genetic recombination was also demonstrated between different strains of bacteriophages by M. Delbruck, W.T. Bailey and A.D. Hershey. These workers were also able to detect and artificially induce mutations in these organisms. Considerable genetic work has since been undertaken by these and other workers using bacteriophages. Drs. A.D. Hershey, M. Delbruck and S.E. Luria shared Nobel Prize in medicine for the year 1969, for their contribution to replication and recombination in viruses.