The unique character of inelastic neutron scattering in the study of magnetic excitations of rare-earth-containing high-temperature superconductors is summarized. It is exemplified for single-ion, cluster and collective magnetic excitations which are directly related to the fundamental interactions associated with the rare-earth sublattice. Crystalline-electric-field and cluster excitations are discussed for which provide information on the electrostatic potential and the magnetic pair coupling at the rare-earth site, respectively. The study of collective magnetic excitations in both grain-aligned samples and single crystals yields more detailed information on the exchange coupling. In particular, we show how Ce doping affects the spin-wave dispersion of , i.e., a strong softening of the excitation spectra is observed which explains the huge Sommerfeld coefficients in (x > 0.1).