Paramagnetic ions and molecules have been exploited quite extensively as extrinsic shift and relaxation probes for investigating the structure and dynamics of biological molecules. The prodigious growth of related research areas is easily discernible as the remarkably widening scope of application in diverse fields in life and material sciences. Sperm whale myoglobin (Mb) is well-known as the first protein to have its three-dimensional structure revealed by X-ray crystallographic study and is also known as one of the first paramagnetic proteins studied by NMR. The heme Fe atom in Mb can exhibit a variety of oxidation, ligation, and spin states. In this chapter, Mb is selected as a reference paramagnetic compound to provide an overview of the relationship between the spectral features and the number of unpaired electrons, because the effects of a change in the spin quantum number S, i.e., the number of unpaired electrons, on NMR spectral parameters of a single compound can be readily understood. Field-dependent broadening of signals of proteins with a series of S values is also described.