From Coherent Tunneling to Relaxation 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...