This chapter discusses the classical detonation theory. The chapter starts with the presentation of the detonation wave as a discontinuity through which the mass conservation, momentum, and energy conservation equations must be satisfied. The theoretical discussion is followed by the presentation of the Rayleigh line and the Hugoniot curve. Afterwards, the Chapman–Jouguet theory is discussed. This theoretical approach is applied successfully to determine the detonation wave velocity by considering that the products are at the speed of sound corresponding to their thermodynamic state. Afterwards, the chapter presents the ZND theory, named after Zel’dovich, von Neumann, and Döring. The ZND theory introduces the idea that a detonation wave is preceded by a normal shock wave, which increases the temperature and pressure of the mixture. The distance between the shock wave and the reaction wave is called the induction length, and it is a characteristic length of a detonation. Moreover, the concept of thermicity and its derivation from classical thermodynamics are also presented. The chapter closes with a formulation of the ZND theory, which accounts for detailed chemical kinetic mechanisms.