The field of ferroelectricity has greatly expanded and changed in recent times. In addition to classical organic and inorganic ferroelectrics, new fields and materials, unknown or inactive 20 to 40 years ago, have appeared. They are important for both basic science and applications, and show technological promise for novel multifunctional devices. New fields include multiferroic magnetoelectric systems, where spontaneous polarization and spontaneous magnetization are allowed to coexist; incommensurate ferroelectrics, where the periodicity of the order parameter is incommensurate to the periodicity of the underlying basic crystal lattice; ferroelectric liquid crystals; dipolar glasses; relaxor ferroelectrics; ferroelectric thin films; nanoferroelectrics. These new fields are not only of basic physical interest, but also of great technological importance, allowing the design of new memory devices, spintronic applications, and the design of electro-optic devices. They are also important for applications in acoustics, robotics, telecommunications and medicine. The book is primarily intended for material scientists working in research or industry. It is also intended for graduate and doctoral students and can be used as a textbook in graduate courses. Finally, it should be useful for anybody interested in following the developments in modern solid state physics.