The research group “Termo-Hidro-Mecánica y Geoquímica de Materiales de Barrera” conforms, along with the Hydrogeochemistry group, the Environmental Geology Unit of the Environment Department of CIEMAT. The activities of this group started in 1989 in the framework of a new research line promoted by ENRESA: the studies related to the deep geological disposal of nuclear waste. The group actively participated in the selection and characterization of engineered barrier materials, that is, the sealing material whose role is to avoid the transport of contaminants to the environment. For this use, clay materials (namely bentonite) were proposed and in this research area the group has reached a high degree of expertise. Currently the activities of this group focus on the characterisation of thermo-hydro-mechanical and chemical properties of barrier materials, both natural and engineered. These studies can be applied in fields other than nuclear waste disposal, since clay-based barriers are used as liners and covers in landfills or in low and very-low activity radioactive waste repositories.