This book describes recent studies of soil behaviour in strong ground motion – one of the most important and problematic issues in engineering seismology. Destructive earthquakes worldwide show us the damage of structures located on soil foundations; therefore, reliable and adequate estimations of in situ soil behaviour during strong motion are needed. The effects of soil response proved to be more complicated and diverse than we thought previously. Additionally to well-known amplification of seismic waves in near-surface softer layers, resonant phenomena and nonlinearity of soil response, during the 2011 Tohoku earthquake, we observed soil hardening and generation of abnormally high accelerations. The author suggests a method for studying these effects in situ, to forecast soil behaviour in future earthquakes. The method is based on processing records of seismic vertical arrays, which are widely developed in the world and should help us in understanding soil response during earthquakes. The book is addressed to a wide range of seismologists, geotechnical earthquake engineers and physicists.