Ribavirin is a very broad-spectrum anti-viral agent used clinically to treat infections by Lassa fever virus, respiratory syncytial virus (RSV) and, in combination with Interferon-α (IFN-α), hepatitis C virus (HCV). Although it was originally synthesized over 30 years ago, the precise mechanisms of its therapeutic activities are still not fully understood. Ribavirin was shown to possess both direct and indirect action mechanisms against several DNA and RNA viruses. These include direct inhibition of viral RNA-dependent RNA polymerases, inhibition of the host inosine monophosphate dehydrogenase, modulation of the host immune response and inhibition of viral capping enzymes. More recently, ribavirin was demonstrated to be able to act as an RNA virus mutagen, increasing mutations in the RNA virus genome and reducing their infectivity. Still the real challenge is to identify which of its biological properties is responsible for the observed clinical efficacy on specific infections. Under this aspect, renewed interest results from its synergistic enhancement of interferon-α (IFN-α) therapy, which could open the way to develop more powerful anti-HCV compounds. This work purpose is to provide a broad overview of all the recognized ribavirin action mechanisms against HCV, which can possibly also explain its synergistic behavior with IFN-α. An overview on the corresponding HCV treatment clinical observations is also provided in the second part of this work.