Abstract: Colloids are widely distributed in groundwater systems and facilitate transport of trace elements and organics. They therefore play an important role in mass transfer with subsequent impacts on groundwater quality. In this contribution, we have summarized the current state of colloid research, including studies of colloid stability, transportation mechanisms, and methods for simulating colloid transport. Our findings suggest that colloid stability is influenced by physical and chemical properties, including the diameter, morphology and zeta potential of colloids, and hydrogeochemical conditions, including pH and the ionic strength of groundwater. Colloid transportation mechanisms in porous media include deposition and straining. Colloid deposition is described as either blocking or ripening because of differences in colloid stability. Current methods for simulating colloid transport are increasingly well-developed, but further methods that simulate the heterogeneity of porous media and associated colloid transport are needed.