  Basic ideas and results which characterize quantum diffusion of defects in
quantum crystals like solid helium as a new phenomenon are presented. Quantum
effects in such media lead to a delocalization of point defects (vacancies,
impurities etc.) and they turn into quasiparticles of a new type -- defectons,
which are characterized not by their position in the crystal lattice but by
their quasimomentum and dispersion law. Defecton-defecton and defecton-phonon
scattering are considered and an interpolation formula for the diffusion
coefficient valid in all interesting temperature and concentration regions is
presented. A comparison with the experimental data is made. Some alternative
points of view are discussed in detail and the inconsistency of the
Kisvarsanyi-Sullivan theory is shown.
