Defects in the quasi-two-dimensional compound NbSe2 were studied by scanning tunneling microscopy. Atomic vacancies as well as substitutional and interstitial defects, which require an accommodation of the surrounding chalcogen atoms, were identified in as-grown crystals. Defects were also introduced after growth, both by heating the crystals and by bombarding their surfaces with Ar+ ions. Heating the crystals for only 2 min to about 470 K causes in addition to Se vacancies local changes in the surface structure with reduced symmetry as compared to that of the bulk. Ar+ ions with energies and doses not exceeding 400 eV and 10(13) ions/cm(2) form large surface craters as well as Se vacancies accompanied by mobile adatoms,