Abstract: This paper comprehensively reviews the microstructure characteristics and hydration mechanisms of montmorillonite from perspectives of its layered structure, water adsorption mechanism on external surface of aggregates and interlayer expansion theories, etc. The results demonstrate that montmorillonite possesses a layered structure composed of two silicon-oxygen tetrahedral sheets and one aluminum-oxygen octahedral sheet. Water adsorption primarily occurs at specific active 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 crystal edge fractures. The external surfaces exhibit strong water adsorption capacity due to negative charges, exposed hydroxyl groups, and exchangeable cations. Attractions (e.g., hydrogen bonding) exist between the crystal layers in addition to van der Waals forces and electrostatic cation-layer interactions. The accumulation of water molecules at the end of the crystal layer can reduce the energy barrier between the crystal layers, facilitating water molecules enter into the crystal layers smoothly. With the Gouy-Chapman theory and modified GCSG models, the interlayer expansion processes can be accurately described.