Liquid crystal (LC) which represent a distinct and unique state of matter exist between solid and liquid phases. They are liquid in mobility and crystalline in their anisotropic physical properties. Apart from their importance in life sciences, liquid crystals (mesophases) have been the novel media in many practical applications, especially in display device technology such as portable devices like wristwatches, pocket calculators, measuring instruments and laptop computers. There are a large number of chemically very different compounds forming different types of liquid crystalline compound. It is an introductory book that deals with the definition of liquid crystals, historical developments, classification, nomenclature, mesophase type and phase structures. In addition, a brief description of the structure-property relationship in the liquid crystalline compounds and the effect of core groups, lateral substituents, terminal substituents and linking groups on the mesophase are also included. Some important applications of liquid crystals are also presented.