This chapter is concerned with the connection between parameters that NMR spectroscopists typically measure and those that can be measured by high-resolution rotational spectroscopy^1–6  or molecular beam resonance techniques,^7–9  all of which ultimately have as their goal the determination of molecular structure. The objective of this chapter is to illustrate these connections and show how NMR spectroscopists can use data from microwave and molecular beam spectroscopy. After a brief discussion of the important and relevant Hamiltonians that connect NMR and molecular spectroscopy, we will illustrate how spin-rotation tensors can be used to establish absolute magnetic shielding scales, provide a better understanding of the relationship between molecular structure and magnetic shielding, and allow theoreticians to rigorously test quantum chemistry computations. Several representative examples will be presented, however, we wish to indicate that this is not meant to be a comprehensive review of the many shielding scales that have been proposed in the literature (the annual reviews of Jameson and De Dios¹⁰  are highly recommended). The discussion of spin-rotation and magnetic shielding tensors is followed by a discussion of how accurate measurements of nuclear electric quadrupolar coupling tensors via microwave spectroscopy or molecular beam techniques can be combined with quantum chemistry computations to provide nuclear electric quadrupole moments, or eQ values. Finally, we will discuss how spin–spin coupling tensors can be characterized using molecular beam resonance techniques and their importance in understanding the mechanisms of indirect spin–spin coupling. The advantage of measuring NMR parameters in the gas phase is that one can eliminate intermolecular effects on NMR parameters.¹¹  Measurement of the temperature and density dependence of NMR parameters via gas phase NMR spectroscopy also allows one to quantify effects of rotational-vibrational averaging.^12,13