This book presents a detailed account of the theory of rotational relaxation of a polar molecule in a nonpolar solvent. The Debye theory, appropriate to noninertial reorientation of a vector located in a spherical molecule, leads to exponential decay of the orientation correlation function with time constant inversely proportional to the rotational diffusion constant. If inertial effects are included, the short-time behavior (high frequency) is modified and this feature shows up in deviations from semicircular shape of Cole-Cole plots.