Abstract: In this paper, the research of achievements and recent advances in on the microstructure characteristics and hydration mechanisms of montmorillonite are comprehensively reviewed and summarized from the perspectives of its layered microstructure, water adsorption mechanisms on external surface of aggregates, water adsorption into interlayer space and expansion theories, etc. Prospects for further study are also outlined. The results demonstrate that montmorillonite has a layered structure composed of one aluminum-oxygen octahedral sheet sandwiched by two silicon-oxygen tetrahedral sheets. Water adsorption firstly occurs at active adsorption sites including hexagonal oxygen rings on silicon-oxygen tetrahedral basal planes, lone electron sites of surface oxygen atoms, exchangeable cations, and valence-imbalanced atoms at the edge fractures of layers. The negative charges of the layer, the exposed hydroxyl group in the hexagonal oxygen ring and the surface cation etc can make the outer surface of the laminate have a strong water molecule adsorption capacity. There are other attractions, e. g., hydrogen bonds between the crystal layers, in addition to the van der Waals forces and electrostatic attractions of cations to crystal layers. Accumulation of water molecules at the end of the crystal layer can lower the energy barrier between the layers, allowing water molecules enter the interlayer space smoothly. The modified GCSG model, which is based on Gouy-Chapman theory, can accurately describe the interlayer expansion of crystal layers.