Progrès récents dans la théorie de la structure spirale

We review some possible forcing mechanisms capable of replenishing the spiral density wave in energy and angular momentum and in particular the case of bars and companions. The case of displacements (e.g. of the center of the bar or buldge from the center of the disk) is also considered. Next we turn our attention to the information that can be extracted from a catalogue of HII regions of galaxies of type earlier than Sc. It is possible to study each angular component (m = 0, 1, 2, ...) separately and find its pitch angle, radial amplitude and phase and that separately for bright and faint HII regions. Finally we discuss departures from axisymmetry in the velocity fields of disk galaxies. In particular we discuss the effect of spiral arms, a central bar and displacements of the bar or buldge from the center of the disk. Applications to particular galaxies are envisaged