A quantum-mechanical approach for determining the electronic structure and atomic configuration of extended defects in metals is described and importance of theoretical methods for interpretation of positron annihilation spectroscopy data is stressed. The electronic structure in defect region is calculated employing the self-consistent Green's function approach based on the tight-binding linear muffin-tin orbital (TB-LMTO) method within the atomic-sphere approximation (ASA). For atomic relaxation, we use (1) the Finnis-Sinclair type many-body central force potentials and (2) a recently developed quantum-mechanical method in which the angular dependence of interatomic forces is properly accounted for. In the latter case, the Hamiltonian is constructed within the TB-LMTO-ASA approach as well. As an example, atomic configuration of the SUM = 5(210)/[001] tilt grain boundary in tungsten is determined and local densities of states at atoms in defect region are discussed.