This book is based on a NATO Advanced Study Institute held to enhance our understanding, at both an experimental and a theoretical level, of the molecular dynamics in liquid crystals. The lecturers at the Institute, each leaders in their respective fields, have contributed chapters to the book with the aim of producing, for the first time, a coherent, pedagogical account of this interdisciplinary subject. The range of materials considered is wide, including lyotropic and thermotropic liquid crystals, biological membranes and polymeric systems. The formalism needed to characterise the rotational, translational and conformational dynamics is developed. Then the use of experimental techniques to investigate the dynamics is described; these techniques include NMR and ESR spectroscopy, neutron scattering, dielectric relaxation, infrared spectroscopy and fluorescence depolarisation. Some of these experiments are influenced by the collective orientations or director modes which are also considered. The results of these experiments are presented and the theory necessary to understand them is described, with particular attention being paid to the influence of the long range liquid--crystalline order on the dynamics.