The paper examines some aspects of the hydromechanical behaviour of argillaceous hard soils and weak rocks. After considering the wide variety of this class of materials, some common behaviour features are sought concerning compressibility, structure development, uplift and swelling, microfabric, anisotropy, effects of discontinuities and hydraulic properties. Those features are examined in the context of appropriate behaviour frameworks developed for this type of materials. Two themes are then selected for special consideration: brittleness and progressive failure and the response of argillaceous weak rocks to deep underground excavations. For the second topic, particular attention is given to field results obtained in deep underground laboratories for nuclear waste disposal. The paper concludes with a summary of the main points.