A nonstationary nonlinear theory of electron-wave interaction has been developed for microwave resonant generators. The system of the integro-differential equations determining the amplitude and phase of the excited oscillations in the high Q-resonant system is obtained. For most cases the solutions of the non-stationary nonlinear interaction equations have been found numerically. However, in certain cases such as in the linear region and in the steady-state of the excited oscillations analytical treatments are developed. Transient performances of the radiation of the electron beam and the estimates for the starting current and the amplitude of the stationary oscillations are presented. The application of the obtained equations for the generator pulse is discussed. The solution for the equation of motion is solved and the oscillation period of the electrons in the wave field is determined analytically.