We discuss the magnetic behavior of rare earth systems where the crystal-field-only ground state of the rare earth ion is a singlet. As exchange increases in such a system, magnetic ordering at zero temperature occurs through a polarization instability of the singlet ground state. Such a process leads to a threshold value for the ratio of exchange to crystal-field interaction necessary for magnetic ordering even at zero temperature. We discuss the magnetic behavior, with regard to both theory and experiment, as this threshold is approached and exceeded. We use the experimental behavior of the rare earth compounds of NaCl-structure with group V elements to illustrate the crystal-field-only behavior, and the behavior as exchange increases up to and beyond the critical value for magnetic ordering.