The high importance of the peroxido-vanadium complexes in many oxidative catalytic reactions has enhanced the research interest in investigating molecular oxygen activation by ligation of O2 on vanadium metal ions. Dioxygen (O2) is a benign, cheap and economically viable oxidant. The activation of O2 by metal ions is a key challenge for modeling biological processes and in applications such as the homogeneous oxidative alkane functionalization, the production of hydrogen peroxide and emerging energy technologies. The activation of O2 by vanadium complexes may involve either coordination and partial reduction of O2 or, in rare cases, the attack of O2 on the organic ligand activated by ligation to metal ion. In this chapter, the advances of dioxygen activation by simple functional vanadium complexes will be reviewed.