Disks develop many sub-structures, either internally, both spontaneously via instabilities and due to the interplay with other components of the same galaxy, or externally, by interactions with other galaxies. In the latter case the fragility of the disk structure poses already interesting constraints on the amount of interaction which can be tolerated. The sub-structures include bars, spirals, rings and warps, each presenting interesting dynamical problems on their own.
Disks extend from the most central to the relatively outer parts of each galaxy, permitting the study of the kinematics and dynamics of the major part of a spiral galaxy using a variety of tracers. Since disks are quite responsive they allow us to probe the different environments in which they reside, from the black hole in their innermost parts, to the halo that surrounds them. The importance of the HI layer to test dark matter is well established, and the connection of the HI and H_2 with the stellar disk yields clues about the history of star formation in disks.
Resonances play a crucial role in the dynamics of disks and it is thus important to locate the main resonant regions. For this, many photometrical and spectroscopical methods have been proposed so far to detect them, but none of them can be applied easily in a systematic way to large samples of galaxies. Hence we still have not reached a global picture concerning their importance.
Star formation is occurring actively in the disk of galaxies. The presence of fresh gas and dust allows new stars to form, sometimes in an organized way due to the response to density wave perturbations in the old disk population. This process can at present be studied with IR-imaging facilities as well as in the classical optical bands.