Les Espaces Acoustiques is a journey, and one might agree with Pierre-André Valade when he calls it initiatory. From a suggestive monodic cantilena (Prologue) we come to the transfigured version (four horns and orchestra) of the same gesture that is nothing more than a stylized image of the sound model that inspired Grisey’s thought up to his last works. The conductor's knowledge of the composer’s poetic ideas and the full sharing of them with his musicians is a fundamental part of the journey that, once the the more technical aspects have been overcome, opens up towards the creation of musical meaning: the true locus of the magic of any successful performance, at the exact moment when the mutual harmony between all the participants involved, including the audience, is achieved. This moment gives meaning to the long journey of study and preparation by both the conductor and the musicians. The traces that these actions leave on the material (score and parts) are the traces of an alchemical process that is part of the very nature of musical performance. They are traces that testify to practices, habits, interpretative visions and methodologies of a work set in a precise cultural context and a specific moment in time that we as a study group have had the privilege of documenting and illustrating through our reflections. We hope that this material will one day serve as a starting point for many other performing and theoretical adventures that a monumental cycle such as Les Espaces Acoustiques will never cease to stimulate.