  We discuss the possible applications of string theory for the construction of
generalizations of the $SU(3)\times SU(2)\times U(1)$ standard model of strong
and electroweak interactions. This includes an investigation of effective $d=4$
dimensional supergravity theories that could be derived from higher dimensional
string theories ($d=10$) and M-theory ($d=11$).It is shown how the question of
unification of gauge and gravitational coupling constants could find a solution
within this framework. The mechanism of hidden sector supersymmetry breakdown
and its consequences for the pattern for soft supersymmetry breaking terms are
discussed in detail.
