  The transition between (non supersymmetric) quantum string states and
Schwarzschild black holes is discussed. This transition occurs when the string
coupling $g^2$ (which determines Newton's constant) increases beyond a certain
critical value $g_c^2$. We review a calculation showing that self-gravity
causes a typical string state of mass $M$ to shrink, as the string coupling
$g^2$ increases, down to a compact string state whose mass, size, entropy and
luminosity match (for the critical value $g_c^2 \sim (M \sqrt{\alpha'})^{-1}$)
those of a Schwarzschild black hole. This confirms the idea (proposed by
several authors) that the entropy of black holes can be accounted for by
counting string states. The level spacing of the quantum states of
Schwarzschild black holes is expected to be exponentially smaller than their
radiative width. This makes it very difficult to conceive (even Gedanken)
experiments probing the discreteness of the quantum energy levels of black
holes.
