An approach to nonlinear dynamics of multimode lasers is developed. It is based on the concept of two systems of eigenoscillations: optical modes and relaxation oscillations. The importance of a correct (not arbitrary) choice of a model is underlined. Characteristic features of two different rate equation models are formulated and compared. A method of selective perturbation on the system is described which makes it possible to study interrelations between optical modes and relaxation oscillations, and to control dynamical behavior of a laser. The possibility of using dynamical regularities for solving both applied and basic problems is illustrated in several examples.