Impurity ions in insulating crystals can be regarded as isolated electronic systems. Their paramagnetic resonance is determined by their ground state characteristics, which can be derived from application of Hund's rules, together with simple applications of ligand‐field theory and consideration of spin‐orbit interaction. The paramagnetic resonance is characteristic of the ion and its coordination in the crystal, and furnishes a powerful diagnostic tool.Fcenters can be regarded as special cases of ions, of particularly simple nature.Vcenters also can be understood in this way, often in terms of molecular ions. Distortions from the full symmetry of the host crystal site, due to the Jahn‐Teller effect, are frequent and predictable. Optical absorption, arising from transitions to higher levels, is closely related to the above properties, and is briefly discussed.