In biomolecules, the time-dependent interactions responsible for spin relaxation are usually i) the dipole-dipole interaction (i.e. the interaction between nuclear magnetic moments), ii) the quadrupolar interaction (i.e. the interaction between the molecular electric field gradients and the electric quadrupole moment of the nucleus, due to the anisotropic distribution of the nuclear electric charge, for nuclear spin quantum numbers greater than ½) , and iii) the chemical shift anisotropy (i.e. the interaction between the nuclear magnetic moment and the magnetic field generated by the electronic distribution around the nucleus) . In paramagnetic systems, i.e. in the presence of unpaired electrons, the interaction between the nuclear magnetic moment and the electron magnetic moment should also be considered, and is often the dominant relaxation mechanism. The dynamic processes modulating all these interactions, and thus causing relaxation, can be of different types, mainly molecular reorientation, internal mobility, chemical exchange, translational diffusion, diffusion on a surface, etc. The time constants describing the persistence of a correlation in the energy values during these dynamic processes are commonly called correlation times.