The microstructure of some soft Swedish clays has been investigated by electron microscopy and described by statistical methods using simple structural parameters. The microstructural pattern was found to be characterized by aggregates coupled by links and groups of small particles. Certain microstructural properties seemed to be related to the geotechnical properties of the undisturbed clay material, such as the permeability and sensitivity. The microstructural changes by consolidation and shearing have been investigated. The results formed the basis of a hypothesis concerning the mechanism of these processes. The aggregates seemed to act like rigid bodies up to a certain stress level, the rigidity probably being connected with their low water content and the very high viscosity of the pore water.