Liquid crystals are complex materials that share properties of both solids and liquids. This is a consequence of complex anisotropic molecules that permit establishing phases with orientational and positional orders. Thus, a large variety of phases and phase transitions can occur in these systems. After a detailed description of general features of these materials, the tensorial nature of the orientational order parameter is discussed. Then, the Landau–de Gennes theory is developed for the isotropic–nematic transition. Later, positional degrees of freedom are included to account for the nematic–smectic transition. Next, the theory is generalized to include fluctuations, distortions and the effect of an external field. In the last part, topological defects are discussed with a particular emphasis on defects such as skyrmions and merons which can form in chiral liquid crystals such as cholesteric and blue phases. Finally, the analogy of these classes of defects with those occurring in non-collinear magnetic materials is considered.