It was nearly a century ago that the viral etiology of sarcomas and leukemia (leukosis) in the domestic fowl was first described by Ellerman and Bang, working in Copenhagen, and Rous in New York. Through the following decades, in an attempt to control these oncogenic diseases which were becoming the main cause of mortality in commercial poultry, extensive investigation in many veterinary laboratories was undertaken. Throughout this period and continuing today, advancements in our understanding of the mechanisms of these viruses are ever expanding. This knowledge has exponentially expanded since the discovery and in turn the development of molecular, nucleic acid-based tools to analyze and interpret these viral genomes. This burgeoning field of research has shed light on countless topics including the elucidation of specific viral mechanisms of neoplastic induction. It has allowed for the creation of diagnostics which are vastly superior in sensitivity and specificity than to their non-molecular-based counterparts, and in the future will lead to the generation of disease resistance in the host. These topics, in specific regard to molecular studies of avian leukosis virus, will be further developed in this thesis.