We discuss the problem of excess noise in lasers of different dynamical classes, as well as adequate models for the description of such noise. It is shown that the excess noise in lasers is caused by mode-mode coupling. The linear coupling leads to the phenomenon of excess noise in the narrow sense (Petermann's excess noise), while the nonlinear mode coupling is the universal mechanism for excess noise of dynamical nature. It is shown that the theory of excess noise in class-B lasers does not require new approaches and can be developed on the basis of models with adiabatically eliminated polarization obtained in a standard way.