The discovery of dark matter should be assigned to the study of rotation curves of spiral galaxies. Indeed, a study that contained clear clues about dark halos was the measurement, in the late 1950s, by means of 21-cm radio observations, of the rotation curve of M31, the Andromeda galaxy. Therefore, this second part begins with a short introduction to galaxies. It starts with a description of how galaxies were discovered, because the topic illustrates a sort of paradigm for more recent discoveries in astrophysics, such as that of Gamma Ray Bursts. It continues with highlights on their morphology, their structure and kinematics, following a description of their typical scales, with special reference to the concept of dynamical time scale. All this should be considered classical information that is a prerequisite for any further discussion about the dynamics of galaxies. The subject of scales gives the opportunity to mention some modern developments about galaxies characterized by unusual properties that are currently studied at the frontier of galactic dynamics. The closing section of the chapter is devoted to introducing the virial theorem in its simplest form, for the discrete and the continuous case; this is an important tool at the the basis of many arguments about self-gravitating systems.