Abstract: This Ph.D. thesis has been devoted to the study of extended defects of α-Ti in presence of interstitial atoms H/O via ab initio simulation. It is divided into three parts. In a first part, the octahedral interstitial site of α-Ti is found energetically more favorable for a H or an O atom. The presence of H decreases the shear modulus G and Young’s modulus E of α-Ti while O has an opposite and stronger effect. H increases the B/G ratio while O decreases it. In a second part two new stacking faults are found. A