An extensive programme of research into the influence of undissolved gas bubbles on the behaviour of fine-grained offshore soils is reviewed. The programme has been based on the development of a laboratory technique for the preparation of reconstituted soil samples containing a uniform distribution of gas bubbles. The structure of these samples is similar to that observed in sediment recovered from the sea bed, and consists of large gas-filled cavities surrounded by a matrix of saturated soil. It is found that surface tension effects limit the difference between gas pressure and pore water pressure, and that the overall void size is effectively a function of the strength of the matrix, so that changes in void volume may be modelled by cavity expansion and contraction in an ideal plastic medium, leading to limits on the difference between gas pressure and mean total stress. A new parameter, operative stress, is shown to influence both the consolidation and the strength of these gassy soils. Thus, during consolidation, the gas volume is controlled by the total stress and the water volume by the operative stress; the undrained shear strength may be increased or decreased by the presence of the gas, depending on the specific values of total stress and operative stress. The operative stress for a gassy soil may therefore be seen as being analogous in its definition (total stress minus pore water pressure) to the effective stress for a saturated soil, but different from the effective stress in that it does not, on its own, control the strains and strength. Gas bubbles are shown to have a major influence on the acoustic behaviour of fine-grained offshore soils.