is an impressive review of ten years of progress in our understanding of interacting tunneling systems. The dielectric and elastic response of (111) off-center impurities in alkalide halides is investigated in detail and the coherent motion of an impurity pair is discussed in view of recently observed rotary echoes. The low-temperature properties of defect crystals with an impurity concentration higher than 100 ppm are reviewed and the dipolar interaction is shown to drive the cross-over to relaxational dynamics. This comprehensive presentation of the theory is illustrated by experimental data for lithium, cyanide, and hydroxyl impurities in various host crystals. This monograph is an ideal reference book for scientists working with defect crystals and glassesweiterlesen