The problem of the determination of distances in astronomy (the so-called problem of the distance scale) is a very old and important problem. About 280 BC Aristarchus of Samos, the famous greek astronomer of the Alexandrian school, already devised a method to find the relative distances to the Sun and Moon in terms of the size of the Earth. Later, Eratosthenes (about 200 BC), another greek astronomer, measured the Earth’s diameter; so the zero-point of this first distance scale was fixed.
Now we are interested by extragalactic distances but the same approach is made : (i) determination of relative distances (ii) determination of the zero-point to obtain absolute distances.
Some distance criteria can be used to determine the zero-point. These criteria cannot generally be used at a great distance. They permit a comparison between galactic objects, like Cepheids, Novae, Supergiants ..., globular clusters..., and the same counterparts recognized in external galaxies. Often application is limited to nearby galaxies. In a first section we will briefly present this kind of distance criteria. For more distant galaxies other criteria must be employed, the zero-point being here fixed with nearby galaxies whose distances are known from the preceding step. We will discuss these criteria in a second section.