Intelligent materials occupy a special place among the huge number of known materials and those, which are being created all the time. Despite the impressive number of publications, no attempt has yet been made to classify compounds with a valence-unstable rare-earth element (R.E.) as smart materials. In this review paper, we show that the valence of the R.E. in some compounds can be changed by external and internal factors, such as temperature, pressure, changes in the occupation of $d$- and $f$-levels, and changes in the coordination polyhedra of atoms. Changing the valence of the R.E. may significantly alter the electrical and magnetic properties of compounds.