[en] We used a special gas-testing apparatus between 1986 and 1989 and obtained a relatively large number of test results within a pressure difference range of 0-100 bar (atm). This seems to have exposed the real features of the gas permeability behavior of concrete, indicating a dependence on pressure differences and aging, and the existence of 'surprising' threshold pressures. The highest pressure difference gradients applied (probably well above anything encountered in practice) were up to 100 bar over a 5 cm thick specimen, or 20 atm/cm (2 MPa/cm). The effect of measurement time, duration of the applied pressure difference, thickness of specimens and pressure gradient used at testing may also be relevant factors in practice. As wet conditions and the long-term behavior of concrete play vital roles in the storage of nuclear waste, this paper focuses particularly on these issues. In continuously wet conditions concrete structures (silos etc.) do not suffer significantly from drying cracks. This report therefore deals chiefly with solid, non-cracked concrete. Crack flow is another complicated subject, and is discussed briefly as a sideline. Two nomograms are introduced illustrating air flow through porous media and cracks. (orig./HP)