Data which indicate that the existence of nuclei and atoms and also of stars and galaxies is very sensitive to the numerical values of the fundamental constants, i.e., the coupling constants of the four interactions and the constants $G$, $\hslash$, $c$, $m_p$, etc., are reviewed. The variation of some constants by a few tens of percent with the other constants remaining unchanged would lead to the disappearance of stable ground states. Possible interpretations of this fact and its connection to some aspects of unified field theory are analyzed.