This introduction presents an overview of key concepts discussed in the subsequent chapters of this book. Metal-organic frameworks (MOFs) have contributed significantly to the fundamental understanding in the field of porous materials, adsorption phenomena, separation technology, and catalysis. The book aims to organize MOFs compounds according to their main components: clusters and linker systems. It contains four parts. First part largely follows the organization of metals in the periodic table, without aiming at a classification since chemical similarity within the periodic table generates fluent passage between certain groups nor aiming for an exhaustive description of all known structures or metals used. The second part outlines essential functions in MOFs. Since shaping of MOFs into granules, monoliths, and nanostructures is a prerequisite for any application, third part addresses those topics. The fourth part reviews the progress in specific characterization methods for MOFs, illuminating long-range and local structural features, and the importance of defect chemistry.