The occurrence, properties and structure of electrically insulating materials arereviewed with a view to the connection between structure and properties. The majorpolarization mechanisms are introduced and the resulting dielectric properties arediscussed. Soft modes and the related topics of displacive and order/disordertransitions are described with example materials systems. Piezoelectric, pyroelectric,and ferroelectric materials are described and the role of crystal symmetry indetermining these properties is examined. Some recent developments in liquid crystalferroelectrics and related organic insulator systems are introduced and selectedaspects of the magnetic and thermal properties of insulators are summarized.