This book is conceived as a lecture course on the theory of dielectric and Kerr effect relaxation in molecular fluids. It is based on seminars and lectures to final year graduate students and students preparing for a Ph.D. The book 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; the 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.