A crucial issue for antiviral therapy is the fact that all antiviral substances rapidly select for resistance; thus, monitoring and overcoming resistance has become a most important clinical paradigm of antiviral therapy. This calls for cautious use of antiviral drugs and implementation of combination therapies. In parallel, efforts in drug discovery have to be continued to develop compounds with novel mode-of-action and activity against resistant strains. This book reviews the current status of antiviral therapy, from the roads to development of new compounds to their clinical use and cost effectiveness. Individual chapters address in more detail all available drug classes and outline new approaches currently under development.