Molecule-based magnets, giant magnetoresistance and spin frustration are just a few terms which demonstrate that our understanding of magnetic behavior, once thought to be mature, has enjoyed new momentum as it is being expanded by contributions from molecular chemistry, materials chemistry and sciences to solid state physics. The spectrum spans molecule-based - organic, inorganic, and hybrid - compounds, metallic materials as well as their oxides forming thin films, nanoparticles, wires etc. New phenomena are explored that open promising perspectives for commercially applied "smart" materials. In topical reviews written by foremost scientists trends and recent advances focusing on the correlations between electronic structure and magnetic properties are explained in a clear and detailed way. With good balance between theory and experiment this book guides advanced students and specialists alike to this research area. It facilitates the evaluation of experimental observations and serves as a base for the design of new magnetic materials. As a unique reference work, it is indispensable for everyone concerned with the phenomena of magnetism!