Abstract
After having listed some of the current problems met in the study of incommensurate phases in insulators, we shall present results obtained during the past five years : Elucidation of the structure of the phase II of biphenyl by spectroscopic methods (inelastic neutron and Raman scattering); properties of C2O4H NH4, ½H2O which displays an incommensurate phase only above a critical pressure; role of impurities, and of the rate of temperature variation, in the evolution of the modulation wave vector.