Viral dynamics is a concept analyzing the time course of treatment-induced changes in blood virion concentration (kinetics) to derive conclusions of where, when and how in the living organism virions are produced or eliminated. Originally applied to human immunodeficiency virus type 1 and hepatitis B virus, it has elucidated mechanisms of antiviral therapy in hepatitis C virus infection as well. This review summarizes key aspects of mathematical modeling as well as important clinical applications, namely induction therapy and prediction of virologic response to treatment. Furthermore, limitations of currently available quantitative assays will be discussed.