Among the magnetic transition metal oxides (MTO), FeO is singular in  its high content of Fe vacancies. Their existence adds difficulties in  the comparison of the measured properties of FeO to the rest of the  MTO, and eases the transformation of FeO to magnetite. Surprisingly,  the existence of Fe vacancies had not been considered within an ab  initio framework until recently. Here we will present our ab initio  results on bulk FeO with a realistic concentration of vacancies. We  will show how defects tend to form clusters that modify the electronic  and magnetic properties of FeO, introducing features that resemble  those of magnetite. We will also show how the tetrahedral environment  linked to Fe defects in the bulk is also important in the  stabilization of the O-termination at (111) surfaces. Furthermore, it  accounts for the puzzling high magnetization measured at reconstructed  surfaces, and seems to play an active role in the stabilizion of  stoichiometric FeO films in the ultrathin limit.