  The first part of this thesis is a general introduction to the bosonic and
fermionic string theory, to the concept of D brane and to string dualities. A
discussion of anomalies cancellation closes the chapter. The second part of the
thesis reviews the basics of orientifold and orbifold constructions and the
Scherk-Schwarz mechanism of supersymmetry breaking. The last part starts with
an introduction to standard cosmology and inflation. Alternatives to inflation
inspired from string theory and time dependent orbifolds are discussed. Finally
time dependent solutions of non-supersymmetric strings are presented.
