The introduction of spin is believed to be a necessary tool if one wishes to quantize general relativity. Then the main problem is to see if the introduction of spin generalizing the general relativity from a geometric point of view, i.e. through the concept of torsion, can be experimentally verified.The reader can find in this book both theoretical and experimental arguments which show the necessity for the introduction of spin, and then of torsion, in gravity. In fact, torsion constitutes the more natural and simple way to introduce spin in general relativity. For that reason it is of...