A new original approach to the study of coordination complexes with pa ra mag netic ions, the modified crystal field theory (MCFT), is represented in the monograph. The approach is based on a new parametrization of the problem by the effective nuc lear charge of a paramagnetic ion. Implicit accounting for ligand electrons signifi cantly enhances the predictive capability of the proposed method. The book gives some examples of the MCFT applications for the in terpretation of various experiments. To describe the spin state variations of pa ramagnetic ions under different coordination complex distortions the spin state diagrams are suggested for the first time.