This text is conceived as a lecture course on the theory of dielectric and Kerr effect relaxation in molecular fluids. It is based on seminars given at the University of Perpignan, Theoretical Physics Group, France and lectures at the University of Montpellier, France to final year graduate students together with third cycle students (preparing for PhD). The general programme of these lectures comprises the following: the rotational diffusion equation; perturbative solution obtained from convolution products; Kerr effect response in pulsed electric fields; rotary friction and diffusion coefficients in the case of partial slip conditions; linear detection; electric birefringence in time-varying fields; study of phase angles; the Fokker-Planck-Kramers equation; role of molecular rotational inertia; and nonlinear dielectric relaxation in coupled electric fields. The literature on the subject originates essentially from review articles which, however, are pitched at such a high level that it is very difficult for students to understand. Moreover, a lot of details are missing from the calculations in these fundamental papers. This book therefore attempts, in a more didactic manner, to fill these gaps.