Viral induced tumors in poultry have considerable economic importance for poultry industry as infection of avian oncogenic viruses causes increased morbidity and mortality of birds during growth and increased condemnation at slaughter. Tumors in commercial poultry are caused mainly by infection with avian herpes and retroviruses. Oncogenic MDV, ALV & REV has been widely disseminated, in spite of the extensive vaccination procedures. It is necessary to diagnose the viruses in the poultry before it poses a serious threat to the public. Classical forms of diagnosis include Histopathology, Immunohistochemistry, Cell culture, ELISA etc., which are time consuming and laborious. The latest and most rapidly advancing form of diagnosing avian oncogenic virus infection involves molecular diagnosis. This book will enable the readers to have an overwhelming idea on development of rapid, cost effective and single step differential diagnosis of avian oncogenic viruses using molecular methods.