Abstract
Studies on incommensurate(INC) phase transition in ferroelectric and ferroelastic crystals are reviewed within the framework of the Landau-theory. The classification of the INC-phases according to the difference of nature of irreducible representations causing a (hypothetical) normal-commensurate transition, and physical properties in the INC-phases are mentioned. As an example, the transitions in [C(NH4) 3] 2CuCl4, whose commensurate phase is ferroelastic are discussed in some details. Collective modes are briefly discussed.