Abstract : In order to study influence of fluctuations on various phenomena linked to heavy ions collisions, a Langevin equation has been derived from a microscopic model. Parameters entering this equation are completely determined from microscopic quantities characterising nuclear matter. This equation has been applied to various physical phenomena at intermediate energies. A first part is devoted to small amplitude collective motions, especially giant resonances. Memory effects in the collisions term of Boltzmann equation have been studied. A formal approach to the influence of the fluctuations on several coupled modes is proposed. A second part is devoted to large amplitude collective motions, where a detailed study of thermal fission is done. Fission rate is calculated and compared to experimental results. Finally, some preliminary works on multifragmentation are also proposed.