| This thesis is devoted to the study of non-perturbative behavior of string theories. The study of non-perturbative states, that saturate the Bogomol'nyi-Prasad-Sommerfield (BPS) bound, constitutes the major part. The rest is on the phenomenology of the non-perturbative effects expected in string theories.; I first reviewed the features of BPS-saturated states. Then, a study of a class of four-dimensional BPS-saturated black-hole solutions of toroidally compactified heterotic string was presented. At the points of maximal symmetry of the two-torus moduli subspace, the dyonic black holes became massless. It led to the possibility of supersymmetry enhancement.; I explicitly constructed a large class of BPS-saturated states in toroidally compactified type II string theory next. They corresponded to orthogonally intersecting BPS-saturated states in ten dimensions. With Kaluza-Klein monopole, they had four charges and preserved {dollar}1over8{dollar} of the N = 8 supersymmetry. I found a simple map to associate each charge with the corresponding Killing spinor constraints. I also explicitly showed how the N = 4 supersymmetries of toroidally compactified heterotic string were embedded into the N = 8 supersymmetries of IIA superstring.; A particular kind of static, non-orthogonally intersecting and non-threshold BPS-saturated states of type II string theories in ten dimensions was then studied. They were parametrised by four independent charges with non-diagonal metrics. The metrics could be diagonalized when one charge was removed. However, the components of the configurations still intersected non-orthogonally.; I started the phenomenological study on the non-perturbative behavior of compactified string theories by investigating the effect of constant threshold corrections on static, non-extreme, and electrically charged dilatonic black holes. Closed form solution for the perturbed black holes was obtained. Then, I studied the cosmological and phenomenological implications of a non-perturbatively induced superpotential in a N = 1 supergravity theory. I used gaugino condensation to fix the dilaton dependence, and T-duality to fix the moduli dependence. Without any fine tuning of parameters, I obtained a supersymmetric vacuum with zero cosmological constant, which could be relevant to the cosmological moduli problem in string theories. |
