This book is devoted to the scientific legacy of Professor Victor Ambartsumian – one of the distinguished scientists of the last century. He obtained very essential results not only in astrophysics, but also in mathematics and theoretical physics. One can recall his fundamental results concerning the Sturm-Liouville inverse problem, quantum field theory, structure of atomic nuclei etc. Nevertheless, his revolutionary ideas in astrophysics and corresponding results are known more widely and have predetermined the further development of this science. The concept about the activity phenomena and objects’ evolution, particularly, determination of the age of our Galaxy, ideas about the stars’ formation nowadays in stellar associations, the activity of galactic nuclei appeared to be exceptionally fruitful. These directions are being elaborated at many astronomical centers all over the world.