Order-disorder phenomena, which play a key role in the energetics, crystal chemistry, and physical properties of numerous solids, may be strongly affected by pressure. The effect of pressure on the state of cation order depends fundamentally on the volume of disordering, ΔVdis = Vdisordered − Vordered. Although ΔVdis may be positive or negative, it is usually positive for cation ordering in rock-forming minerals; pressure, therefore, tends to increase order in geological systems. Three structural mechanisms that contribute to ΔVdis, are (1) positive ΔVdis(typically ≤1%) associated with ordering of similar cations over two or more similar sites, (2) variable ΔVdis(−1% ≤ΔVdis≤ 1%) associated with Al-Si tetrahedral ordering in framework minerals, and (3) large ΔVdis (up to 5%) associated with cation ordering involving mixed valence, mixed coordination, or both. Pressure alters order-disorder reaction rates by restricting cation motions or changing cation diffusion mechanisms. Influences of pressure on ordering in olivines, orthopyroxenes, high-pressure ferromagnesian silicates, spinels, garnets, perovskites, and rhombohedral carbonates are discussed. Several experiments are proposed to elucidate the phenomenon of high-pressure ordering in systems of mineralogical interest.