[en] String theory has been proposed as the ultimate unified theory which provides a finite theory of quantum gravity. Although only a few consistent string theories are known to exist, a large number of solutions have been constructed. At the current stage of development one cannot decide from first principles which solution should be realized in the physical world. This thesis describes an analysis of the problem which leads to a drastic reduction in the number of acceptable solutions, and an investigation of the only model known to satisfy the constraints. The author imposes two constraints any acceptable model should obey: (1) the model must have spacetime supersymmetry; and (2) it must contain chiral fermions. The effective theory at energies below the Planck (string) scale should be a Grand Unified Theory (GUT). Unified theories are built on two basic assumptions: (1) the unified gauge symmetry breaks to the standard model gauge symmetry at a suitable high-energy scale; and (2) the model should remain perturbative at all energy scales. Symmetry breaking is achieved in most models through the standard Higgs mechanism by letting scalar fields in the adjoint representation of the gauge group acquire vacuum expectation values. It is a result of the calculations that either of the constraints alluded to above eliminates all adjoint scalars from the physical spectrum. The author has translated the second GUT restriction into a tree-level unitarity condition, and devised a method to systematically derive unitarity bounds on any unified model. He has applied this method to the flipped SU(5) x U(1) model and found bounds on several model parameters