When a spin ensemble is perturbed from thermal equilibrium, it may return to such a state via a variety of incoherent mechanisms. This process is referred to as relaxation, the theory of which has been of intense interest in the fields of nuclear magnetic resonance (NMR), among others. This thesis considers the cross-correlated mechanisms of second-rank interactions, restricting the treatment to the intrapair dipole-dipole and symmetric chemical shift anisotropy.After reviewing the mathematical and quantum mechanical requirements, our focus is directed to spin relaxation theory, and specifically on how one may describe the effects of anisotropic rotational diffusion of the host molecule has on the spin system. This is applied to the description of spectral line shape in a near-equivalent system exhibiting asymmetric broadening in its NMR spectrum. This leads naturally to consider longitudinal and singlet-order relaxation.