This book is the eighth in a series of Proceedings for the S´ eminaire Poincar´ e, which is directed towards a large audience of physicists and of mathematicians. The goal of this seminar is to provide up to date information about general topics of great interest in physics. Both the theoretical and experimental aspects are covered, with some historical background. Inspired by the Bourbaki seminar in mathematics in its organization, hence nicknamed “Bourbaphy”, this Poincar´ e SeminarisheldattheInstitutHenriPoincar´ einParis,withcontributionsprepared inadvance.Particularcareisdevotedtothepedagogicalnatureofthepresentation so as to ful?ll the goal of being readable by a large audience of scientists. This new volume of the Poincar´ e Seminar series “The Spin” corresponds to the eleventh such Seminar, held on December 8, 2007. It describes how this once mysterious quantum reality called spin has become ubiquitous in modern physics from the most theoretical aspects down to the most practical applications of miniaturizing electronic and computer devices or helping medical diagnosis.