Abstract
A non-exhaustive survey of the influence of extrinsic defects (such as dislocation lines, substitutional or interstitial impurities and precipitates) on the physical properties of incommensurate systems is presented; special attention is paid to incommensurate insulators having a displacive modulation. Direct-imaging techniques such as X-ray Topography and Transmission Electron Microscopy are shown to be very fruitful tools in order to characterize the texture of the phase front at the lock-in transition, the mechanisms of the modulation wave-vector variation with temperature and the pinning action exerted by extrinsic defects of the crystal.