The Law of 30 December 1991 Loi n¯ 91-1381 du 30 dÇcembre 1991 relative aux recherches sur la gestion des dÇchets radioactifs. conferred on Andra the task of assessing the feasibility of a high-level, long-lived waste (HLLL waste) repository in a deep geological formation. This volume of the Dossier 2005 Argile reports on the results of the study from the standpoint of the architecture and management of such a repository. It is based on the characteristics of the clay formation studied in an underground research laboratory located in the Meuse and Haute-Marne departments.