This article reviews experimental and theoretical work on the spectrum of ice. Its features in the radiofrequency region arise from the reorientation of water molecules at lattice sites where strong thermal excitations of a molecular vibration produce a defect. The large permittivity and relaxation time in ice are consistent with our knowledge of its structure. The microwave region has been less extensively studied, but enough information is available to determine how the molecular polarizability of a water molecule in ice depends upon temperature and volume. In the infrared region, two features, arising from the translational lattice vibrations and symmetric O-H stretching, have received much attention, and progress is being made towards understanding them theoretically. The visible and ultraviolet regions show no distinctive features, and the absorptivity in this region is small.