A fairly complete account of both chemical and physical properties of liquid crystals can be found in the books by G. W. Gray, P. G. De Gennes and S. Chandrasekhar, which have already become classic. This chapter outlines the phenomenological premisses upon which the following chapters are built. Liquid crystals are mesophases, that is intermediate states of matter, which flow like nearly incompressible viscous fluids, and yet retain several features, especially optical, characteristic of crystals. In 1922 G. Friedel proposed to classify liquid crystals into three wide categories, which he called nematic, cholesteric and smectic. The appearance of the nematic phase in both thermotropic and lyotropic liquid crystals results from the delicate equilibrium between two contrary tendencies, the one in favour of orientational order and the other against it. While in a nematic liquid crystal even a faint order must reign among the molecular long axes, no order whatsoever is demanded of the centres of mass of the molecules.