This chapter discusses experimental methods used in the investigation of point defects and their interaction at low temperatures. The experimental methods are extended X-ray absorption fine structure (EXAFS), inelastic neutron scattering (INS), mechanical relaxation (MechR), magnetic relaxation (MagR), change of the elastic constants and Mössbauer spectroscopy (MS). EXAFS can be used only for the investigation of self-interstitial atom (SIA) or V (vacancy) that are trapped at an solute atom (SA) (probe atom); however, only very few systems have been investigated so far. Inelastic neutron scattering studies provide the most direct information on special vibrational modes present in the phonon spectrum. For the defect concentration obtainable in the experiments the effects of the resonant modes on the phonon spectrum and the phonon dispersion curves are small. Anelastic relaxation involves the thermally activated stress-induced ordering of defects whose symmetry is lower than that of the host lattice. Two common techniques exist for studying mechanical relaxation, elastic after-effect measurements and internal friction measurements. internal friction and modulus change are sensitive techniques for the investigation of the interaction of point defects with dislocations that undergo an oscillatory shear stress. The principles of magnetic relaxation techniques as applied to ferromagnetic metals are similar to those of mechanical relaxation techniques. The elastic constant change can be observed experimentally by the use of the same equipment as employed in the mechanical relaxation studies. In the Mössbauer measurements new “defect lines” appear in the Mössbauer spectrum, whose shift and/or splitting is characteristic of a given complex structure.