Fluoro substitution in thermotropic liquid crystals provides a general way of modifying the properties of a parent system. Melting point, transition temperatures, mesophase types and other physical properties are affected by fluoro substitution so that frequently the behaviour of the parent compound can be manipulated and improved in a predictable manner. This review discusses the effects of fluoro substitution at different positions in a variety of core systems and briefly considers the problems arising in the synthesis of fluoro-substituted liquid crystals.