It is not so easy to give an Introductory Report on a topic like that, where during the last years new empirical material grew rapidly, a topic which is still too young for a consolidation of our knowledge. I must therefore concentrate on some partial questions which, according to my opinion, seem of some importance. When we speak now of extremely young variables we mean, of course, the evolutionary age (which is very short) and not the absolute age of the objects. In this way a B-type star of 106 years is no longer young, contrary to a K- or M-type star of the same age. Let us therefore call those stars very young ones which during their evolution have not yet reached the age zero main sequence, that is to say the gravitationally contracting stars.