Marek’s disease (MD) is one of the most common lymphoproliferative diseases of chickens which results in mononuclear cell infiltration of one or more of the visceral organs and nerves. Marek’s disease has a tremendous economic impact, firstly because of continuing losses due to the disease and secondly because of cost of vaccination. The annual losses due to this disease world over have been estimated at more than one billion US dollars. The vaccination can prevent formation of tumor but generation of infectious virus is not prevented. Despite the widespread and successful use of vaccines for the last 40 years, MD virus (MDV) shows a continuous evolution of virulence.It becomes mandatory to study the molecular characters of newly evolving virulent strains and to conduct pathotyping assay to find out magnitude of virulence in each current field strains. This book will throw light on oncogene characterization and pathotyping of field isolates of Marek's Disease Virus.