Aspects of non-supersymmetric strings theories or with broken supersymmetry

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.