This paper deals with clay microstructure studies using electron microscopy techniques. It focuses on recent developments of SEM and TEM when these are used to study clay microstructures for a better understanding of hydration, structural stability and rheological properties of clayey soil systems. The following aspects are successively examined, (1) clay texture and (2) its consequences on hydration and shrinkage; (3) clay microstructure and its relation to structural stability and hydraulic conductivity and (4) the role of clay-associated substances in soil physical properties.