The microstructure of a number of undisturbed clays has been studied in the scanning electron microscope. These clays represent well known deposits from various parts of the world and can be classified under the following groupings: post-glacial marine clays, post-glacial brackish and freshwater clays, overconsolidated stiff fissured clays, and collapsing partly saturated soils. The object of the study is to relate the geological history of the soil to the observed engineering behaviour. It is seen that in general the most important factor governing microstructure is the chemical and physical environment obtaining during deposition, although the conditions resulting in flocculation and dispersion during natural sedimentation are not yet fully understood. Subsequent stress changes and weathering play an important role in modifying the microstructure, and particularly in developing macro-structural features such as fissures. In certain soils it is this macrostructure which dominates engineering behaviour in the mass; but in many cases such as sensitive clays and collapsing soil it is the microstructure which provides the important features.