The discovery of high-Tc superconductivity in the perovskite-type copper-oxide compounds has given rise to a large amount of materials research. Of particular interest in determining the nature of the superconductivity is the effect of substituents at various sites in these compounds. It has been realized that the superconducting transition temperature is essentially unchanged upon replacing the Y and La ions by “magnetic” rare-earth (R) ions. This surprising observation is in contrast to conventional super-conductors, for which paramagnetic ions usually have a large detrimental effect on superconductivity. It is therefore important to achieve a detailed understanding of the low-energy electronic properties which define the magnetic ground state of the R ions. In particular, information on the crystalline-electric-field (CEF) interaction at the R site is highly desirable, for the following reasons: For many high-Tc compounds superconductivity and long-range three-dimensional magnetic ordering of the R ion sublattice coexist at low temperatures. An understanding of both the nature of the magnetic ordering and its apparent lack of influence on Tc requires a detailed knowledge of the CEF states of the R ions. In most high-Tc compounds, the R ions are situated close to the CuO2 planes where it is widely believed that the superconducting carriers are located, thus the CEF interaction at the R site constitutes an ideal probe of both the local symmetry and the charge distribution of the superconducting planes and thereby monitors directly changes of the carrier concentration induced, e.g., by oxygen nonstoichiometry, pressure and disordering effects. The temperature dependence of the line widths of CEF excitations provides information on the supercon ducting energy gap.