  We study cosmologies based on low-energy effective string theory with
higher-order string corrections to a tree-level action and with a modulus
scalar field (dilaton or compactification modulus). In the presence of such
corrections it is possible to construct nonsingular cosmological solutions in
the context of Pre-Big-Bang and Ekpyrotic universes. We review the construction
of nonsingular bouncing solutions and resulting density perturbations in
Pre-Big-Bang and Ekpyrotic models. We also discuss the effect of higher-order
string corrections on dark energy universe and show several interesting
possibilities of the avoidance of future singularities.
