Microwave spectroscopy and molecular beam resonance methods provide a wealth of information about NMR parameters. For example, nuclear spin rotation tensors provide information about the absolute values of magnetic shielding experienced by nuclei in isolated molecules. Molecular beam resonance methods are capable of yielding both direct and indirect nuclear spin–spin coupling tensors, fundamental data difficult or impossible to obtain by any other method. Finally, electric field gradient tensors at quadrupolar nuclei in isolated molecules are provided by high-resolution rotational spectroscopy. Several examples illustrating the importance of the connection between these spectroscopies are presented. The precise data from microwave spectroscopy and molecular beam resonance methods, together with gas phase NMR data, are also being used as a benchmark to test computational quantum mechanical procedures. Recent progress in this area is allowing scientists to better understand the role of relativistic effects in the interpretation of NMR parameters. Again, several examples from the recent literature are presented.