Our present knowledge about stellar evolution is still so limited that, in presenting the basic facts, I wish to restrict myself to a discussion of a very simple but fundamental question.

The question is this. Are we today in a position to point out two groups of stars, one which in terms of the cosmical time scale is young, the other which is old? The two groups of stars which I want to discuss are the O- and B-type stars of high luminosity as an example of young stars and the stars in globular clusters as an example of old stars.