Over the last years a considerable number of effects have been found in Rare‐Earth (RE) systems which are caused by the crystalline–electric field (CEF) splitting of the RE‐ions. This splitting is usually of the order of 1–100 cm−1and involves the ground stateJ‐multiplet of the incomplete 4f‐shell. A discussion will be given of some of the observed phenomena by using simple physical arguments. One group of effects concerns the influence of the CEF‐split ions on the transport properties of conduction electrons in case the matrix is metallic. The behavior of the thermoelectric power is here of special interest. Furthermore if the matrix is a superconductor the CEF‐splitting shows up in the ability of the RE‐ions to suppress superconductivity. A second group of effects concerns the coupling of lattice deformations (phonons) to CEF‐levels. Special attention will be paid to phenomena which involve an applied magnetic field. Examples are the Faraday rotations of the phonon‐polarization vectors or the Cotton‐Mouton effect. Analogous effects occur for optical phonons. They can be detected by Raman‐scattering experiments.