[en] The theory of self-interstitials in metals is reviewed. First the trends as shown by computer simulations are discussed with respect to the energies and structure of interstitials in fcc and bcc lattices. The vibrational behavior is treated in detail, with emphasis on the occurrence of low frequency resonant modes due to the strong repulsion of the atoms in the surrounding of the interstitial. Local frequency spectra for the interstitial atoms are presented for the 100-dumbbell (fcc). The resonance modes of interstitials lead to anomalies e.g. in Debye-Waller factor, specific heat, elastic constants and phonon-dispersion curves, some of which have been observed experimentally. The structure and properties of di-interstitials are treated briefly. Further the occurrence of mixed dumbbells consisting of one impurity atom and one host atom and their dynamics is discussed