The knowledge of the properties of colloidal dispersions is fundamental for designing and optimizing the usage of clays and clay minerals. The colloidal behaviour of these dispersions is very complex due to the anisometric (and often irregular) particle shape, the anisometric and pH-dependent charge distribution, the variable particle dimensions as a consequence of swelling, delamination and exfoliation, and the ion-exchange properties. Therefore, this chapter gives information on the structure, charge distribution, structure of the hydrates, diffuse ionic layer, and the interactions between the colloidal clay mineral particles (electrostatic, van der Waals, ion correlation, steric stabilization). A large section refers to the coagulation of clay mineral dispersions by salts, the influence of organic compounds, and the destabilization (flocculation by bridging or charge neutralization) or stabilization by polymers (by recharging or steric stabilization). In a further section is described the aggregation of clay mineral particles leading to different types of sediments (decisively determining sealing, plastering, stirring, filtration processes, plasticity) or resulting in gel formation, often with thixotropic properties. Also mentioned is the preparation of colloidal metal (hydr)oxides and sulphides within the network of clay mineral particles or even between the clay mineral layers.