Nonlinear interactions of a large number of cavity modes are responsible for practically important characteristics of high power multimode fibre lasers, such as spectra of generated radiation. The total generated fibre laser power is distributed between multitude of resonator modes, each with a small amplitude. However, the overall nonlinear (powe dependent) effect of the interactions between such small amplitude modes is easily observable. The nonlinear Kerr effect in fibre affects the propagation of light in the cavity and leads to a nonlinear four-wave mixing of longitudinal modes. As a result, the resonator is not linear and the operation and performance of such lasers depends on radiation power. For instance, the spectra of generated radiation experience spectral broadening as the laser power increases. Fibre laser is not only a remarkable engineering device, but also a complex nonlinear physical system with a rich repertoire of dynamic behaviour and phenomena. Mathematical models of the wave tubulence in fibre lasers and open questions will be disucssed.