Measurement of 1 5 N relaxation times provides information both on the rotational diffusion tensor of a protein and on its internal backbone dynamics. It will be shown that the orientation of the diffusion tensor can be obtained in a straightforward manner provided the structure of the protein is known. Results obtained for ubiquitin (8 kDa) and the HIV protease dimer (22 kDa) yield an axially symmetric and an asymmetric diffusion tensor. Both are in excellent agreement with results from hydrodynamic modeling on these two proteins, provided flexibility of the backbone is taken into account.Proteins frequently have a net magnetic susceptibility anisotropy, resulting primarily from aromatic sidechains and/or paramagnetic metals. As a result of this anisotropy there is a small degree of ordering of the protein in solution, which increases with the square of the applied magnetic field. As a result, in high magnetic fields dipolar couplings do not average to zero and the residual dipolar coupling provides information on the orientation of the dipolar interaction relative to the susceptibility tensor of the protein. Methods have been developed which can, in favorable cases, measure such interactions with a precision of 0.02 Hz. Applications to structure determination will be discussed.