The Communist Party appeared a hundred years ago on the French political and social scene. According to opinions and moments, it has been the party of Moscow, of those shot, of the working class, of the union of the left, the party of the foreigner or that of the nation. It has been underground, in government, in town halls, in factories or in the streets. Some considered it too revolutionary, others not enough. More than others, it aroused passions, positive or negative. It attracted many and repelled just as many. After the fall of the USSR, it decided to remain a communist party, while many others gave it up. But it no longer has the place it once had, in reality as in the imagination. This book does not intend to judge, but to provide keys to understanding. It is based on a considerable number of archives that are now available and is an ordered and distanced look at an object that is not lacking in complexity and no doubt even in mystery. This book has been translated from French to English thanks to a financial help from the Gabriel Peri Foundation and the LIR3S UMR Cnrs 7366 of Dijon.