  There are situations in string theory when a finite number of string quanta
induce a significant backreaction upon the background and render the
perturbation theory infrared-divergent. The simplest example is D0-brane recoil
under an impact by closed strings. A more physically interesting case is
backreaction on the evolution of a totally compact universe due to closed
string gas. Such situations necessitate qualitative amendments to the
traditional formulation of string theory in a fixed classical background. In
this contribution to the proceedings of the XVII European Workshop on String
Theory in Padua, I review solved problems and current investigations in
relation to this kind of quantum backreaction effects.
