Recent development of studies on phase transitions in hydrogen-bond crystals are reviewed from a theoretical point of view. These crystals have been considered to display order-disorder type behaviors in phase tansitions at atomospheric pressure. After recent high pressure studies on these crystals, transition temperatures tend to zero kelvin with increasing pressure. If disordered arrangements of radicals in their directions hold down to zero kelvin, these facts are inconsistent with the third law of thermodynamics. A possibility of a displacive type is discussed in the mechanism of phase transition in these crystals.
