EN
The theory of electron correlation phenomena is reviewed and used to interpret the transport, thermal, or magnetic properties of V_{2}O_{3}, LaNiO_{3} and Fe_{3}O_{4} systems, each of which to a greater or lesser degree displays correlation effects. A rationale is provided for interpreting the various types of phase transitions encountered in the vanadium sesquioxide and magnetite host systems and to show why these are altered by small compositional variations.