In December 1977, a strange word entered our everyday lives: RER. The idea was revolutionary at the period: inventing a wide-gauge metro that crossed Paris and the Paris region at high speed from east to west and from north to south and directly connected it to many districts in Paris. The RER incarnated French modernity in the same way as the CNIT or later the TGV. It gave the Ile-de-France its current physiognomy and is reinventing itself daily to face up to its tremendous success. This book presents the challenges of this history, political as well as sociological and cultural. Its goal is to shed light on the almost "intimate" relationship of passengers to the RER which so closely touches our lifestyles, our daily experiences, even our language. At the approach of Grand Paris, whose backbone the RER will be, this book draws a passionate portrait of a region not like any other through narratives, illustrations, plans, photographs and extracts from films and books. In the end, Ile-de-France residents or not, we all belong to the RER generation !