Liquid crystals represent a special (fourth) state of aggregation, assigned a place between liquids and solid crystals; hence they are also called mesophases. They combine the flow properties of a liquid and a molecular order comparable with that of a crystal. Liquid crystals are of interest as a research topic in physics, chemistry and biology and have applications in medicine for diagnostic skin thermography, in nondestructive testing of materials and particularly in optical devices. Annual production of liquid crystal displays runs into the millions. The "Handbook of Liquid Crystals" differs from other treatises available on this topic in providing a documentary account of the subject. Thus the main emphasis is placed on complete coverage, comprehensibility and fast retrieval of desired information rather than on specific detail. This leads to various singularities of presentation and subdivision; for instance, the literature references (numbering over 8000) include the title of the publication cited (usually in abbreviated form in English, German, French, or as an English translation) and, whenever traceable, the appropriate reference to Chemical Abstracts.