In the framework of the law of December 30, 1991, France is studying the feasibility of high level vitrified waste (HLW) disposal in deep geological formations. The reference disposal concept is based on a multibarrier system (waste package, steel overpack, clay engineered barrier, geological barrier). During the disposal lifetime, gases could be generated (mainly H 2 issued from anaerobic corrosion of steel) and could disturb the near field around waste packages. To confirm the stability and long term safety of the disposal, it is necessary to know the impact of gases, in particular on the clay engineered barrier. Recent studies carried out by CEA on gas migration in Fo-Ca clay (candidate material for the engineered barrier) are presented here. Main results concern the H 2 permeability of clay taking into account its dry density as well as its water saturation degree. Gas breakthrough phenomenon is also investigated. From a general point of view, permeability data are well organised: the higher the dry density and water saturation degree, the lower the H 2 permeability. Concerning the behaviour of compacted Fo-Ca clay under high H 2 pressure, two kinds of gas transport threshold pressures, critical and breakthrough pressures are determined. The critical pressure (or gas entry pressure) is the gas pressure under which no gas flow is detected. The gas breakthrough pressure is detected when the gas flow rate and gas permeability show a drastic change. In accordance with updated interpretations, this pressure is associated to high conductivity pathways aperture in the compacted clay. The critical and breakthrough pressure concept is essential to understand gas migration in compacted clays. It allows to determine pressure domains for which gas migration will be possible without damaging the clay engineered barrier and those for which major discontinuities could occur in the material, altering its performance.