HBV and HCV infections represent the main cause of chronic liver disease worldwide, affecting 350–400 million and 170 million people, respectively. They share common modes of transmission. The currently recommended therapy for chronic HCV is the combination of pegylated interferon alfa and ribavirin. This book describe in brief the biological properties, epidemiological data, clinical manifestations, diagnostic methods for the two catastrophic agents (HBV & HCV) and discuss the possible impact of HCV on the emerge of Occult HBV in chronic HCV patients. Understanding of the interaction between HCV & HBV may influence prognosis and the treatment strategy which lead finally to the benefit of the patients in both healthy and economical sides. This book is targeted those interested in the field of virology.