This state-of-the-art report starts by drawing a broad picture of gas transport in the specific context of geological disposal. The role of clay barriers in repositories is explained. An overview of relevant clayey materials and their properties is given, focusing on the materials studied in the experimental and modelling programmes of the WP GAS of EURAD. Then, the report details the main gas transport mechanisms which are expected to take place in a disposal system in the post-closure phase: (i) the diffusive transport of dissolved gas, including retardation effects by physico-sorption of gas on the solid phase, when gas produced within the system can dissolve in the pore water and be transported by diffusion; (ii) visco-capillary flow and (iii) the formation of discrete gas-filled pathways, if a free gas phase develops as the gas production rate exceeds the rate at which gas can be dissolved and evacuated by diffusion. The consequences of the modes of gas transport are discussed in terms of temporary or permanent disturbance of porous media. The hydro-mechanical processes associated with formation of additional pathways through the materials, such as pathway dilation and the creation of discrete pathways, are described in detail. Beyond the disturbances of the materials by the different gas transport modes, the likely consequences on barrier integrity are examined. Finally the conceptualisations of gas transport at the scale of disposal systems and approaches that can be used for evaluating the impact of gas with respect to the functioning of different repositories are discussed. At each stage of the description of gas transport mechanisms, particular attention was paid to emphasising the interests and needs of the end-users in the context of geological disposal, as well as to highlighting the common understanding of these mechanisms and listing the knowledge gaps as these stood at the beginning of the WP GAS of EURAD. The compilation of all the research on the transport of gas in clayey materials detailed in this SOTA reveals a great diversity of experimental results in terms of observed phenomena and processes. The comparison of results (and experiments) is not trivial and different interpretations are sometimes possible to explain the observations of the same experiment. The lack of reproducibility and/or replication further compounds the experimental uncertainties. Next to their consolidation, the output of laboratory tests need to be transposed to the conditions, in particular the stress fields, prevailing in a deep geological repository. With respect to modelling of gas transfer in saturated clay materials, continuum and discrete approaches exist. For both, the transition from the laboratory to the in situ scale is still a challenging task. It is the ambition of the WP GAS to address these issues. This SOTA is thus a baseline for the research activities carried out within the WP GAS of EURAD and a yardstick to measure its progress.