Verification and validation (V&V) are the primary means to assess accuracy and reliability of computational simulations. V&V methods and procedures have fundamentally improved the credibility of simulations in several high-consequence application military areas, such as, safety of nuclear weapons, and the trial of some missiles. In this paper we discuss how to quantify the V&V process for a better description of the model and simulation (M&S). After a representation of the concept of V&V in the beginning, we give the mathematical description of M&S, the quantification of V&V in unit models, and then the hierarchy of V&V.