Broadly speaking, crystalline molecular organic materials (MOMs) are soft solids with a 3D periodic distribution of organic molecules exhibiting weak intermolecular forces, their cohesion being essentially mediated by dipolar (permanent or fluctuating charges), hydrogen bonding and π–π interactions. The molecules involved in the formation of such materials may be of a purely organic nature (e.g., metal-free) or based on hybrid organic–inorganic combinations (e.g., organo-metallic with transition metals). Solids can be built from molecules of a single species or binary or ternary combinations, and inorganic molecules can also be introduced forming hybrid organic–inorganic materials. Such regular solids are often seen by chemists as supramolecular entities, the solid as a macromolecule, in spite of their discrete character, while physicists tend to think in terms of a weakly interacting ordered gas, with cohesion energies larger than about 0.2 eV per molecule the typical energies of noble gas crystals. Although describing the same objects, the terminology used by chemists and physicists is usually distinct and scientific discussions are not always fully synthesized, hindering the desired effective flow of ideas. One of the objectives of this book is to bring both scientific communities to a common neutral playground to better understand the inherently interdisciplinary, rich, complex and exciting field of crystalline MOMs, which involves both experimental and theoretical chemists and physicists and material scientists.