It has long been appreciated that the rotational diffusion coefficient (Dr) and hence the rotational correlation time (τr) of a transmembrane protein about its membrane normal axis is predicted to be highly sensitive to its cylindrical radius (e.g., Saffman and Delbruck (1)). Due to the highly viscous nature of a cellular membrane or a membrane bilayer, the correlation times for most transmembrane proteins are predicted to be in the microsecond or longer correlation time range. This time range is not readily accessible to classical spectroscopic techniques like time-resolved or frequency domain fluorescence anisotropy when using conventional fluorescence probes or by continuous-wave electron paramagnetic resonance (EPR) when using nitroxide spin labels. However, as shown by the early work of Hyde and Dalton (2) and in the seminal study by Thomas et al. (3), the range of motional sensitivity of EPR could be extended into the microsecond-to-millisecond time range by saturation transfer EPR (ST-EPR) spectroscopy using conventional nitroxide spin labels. Since its introduction, ST-EPR has been utilized to study the very slow rotational motions of a wide range of membrane proteins and other large protein assemblies.