This volume brings together recent developments in quasispecies theory extended to variable environments and practical applications in elucidating viral dynamics and treatment designs. In particular, the existence of an error threshold in rugged fitness landscapes has opened the way to a new antiviral strategy termed lethal mutagenesis, which is now under intensive theoretical, experimental and clinical investigation. As such the book explains how an understanding of quasispecies dynamics within infected organisms has increased our knowledge of viral disease events. From a clinical perspective, population dynamics highlights important problems for viral disease control, such as the selection of drug-resistant mutants that often accompanies treatment failures, and suggests means of increasing the effectiveness of antiviral treatments. The book is intended for students and scientists interested in basic and applied aspects of biophysics, chemistry, biology, evolution and medical virology. .