Solid/liquid separation is concerned with the removal of molecules and molecular aggregates, colloids and particulates from liquids. Interaction forces can make the separation easier or more difficult. The nature of the dominant interactions varies according to the type of dispersed phase, and according to the properties (for example, ionic strength) of the continuous phase. Traditional approaches to modelling filtration processes, and hence to design methodologies, fail to incorporate any of the relevant forces that exist in fine particle separations. However, effects of the interactions are interpreted through the use traditional approaches combined with experimental data. Future theoretical developments need to be better based on the fundamentals of solid/liquid systems. Particle or solute interactions with the separating surface are important. In cake filtration these determine the ease of cake discharge, or in crossflow filtration they determine the propensity of a surface to foul. The separating surface can take many forms - woven textile media, sinters or felts, for example - and can be made from a wide variety of materials, such as polymer films, synthetic or naturally occurring fibres, metals, ceramics or glass. The paper discusses the general nature of mechanical and thermodynamic interactions between these media and the dispersed phase.