This chapter is dedicated to the simplest of the orientational ordered mesophases: the uniaxial nematic. This phase appears usually directly below the isotropic liquid (except for a few cases when it is present between two more ordered phases, a feature known as re-entrance) and flows like a simple liquid, with macroscopic viscosities of the order of 1 poise for both thermotropic and lyotropic liquid crystals. Observation under the polarizing microscope of a free droplet placed on a slide reveals that the phase is birefringent and strongly diffuses light, as can easily be deduced from the characteristic “twinkling.” Thermal fluctuations are therefore important. In larger drops, one can equally observe characteristic thread-like structures (see figure A.II.1) representing linear discontinuities of the optical axis. These defects, termed disclinations, are the analogue of dislocations in solids and shall be described in chapter B.III.