The first half of the book presents the effective field approach, which is certainly the simplest approach to investigate cooperative phenomena and the accompanying phase transitions. It provides a common, intuitive approximation to the physics of such diverse phenomena as liquid-vapor transitions, ferromagnetism, superconductivity, order-disorder in alloys, ferroelectricity, superfluidity and ferroelasticity. The connection between the effective field approach and Landau's phenomenological theory is stressed. The latter half is devoted to some specific applications of the effective field concept to ferroelectric systems.