Proteins known as RGS proteins (Regulators of G Protein Signaling) regulate G protein signaling by binding to Ga. For the RGS protein RGS10, this regulation is assisted by the ubiquitous calcium signaling protein calmodulin (CaM), although the precise mechanism is unknown. This alludes to a potentially intricate mechanism for Ga regulation by RGS10 and CaM, perhaps with CaM serving to mediate the interaction of RGS10 and Ga in a ternary complex. In an interesting turn of events, a second function for RGS10 was recently demonstrated, which is to regulate signaling by a protein known as STIM2 (Stromal Interaction Protein), although a direct interaction has not been demonstrated. Interestingly, CaM also regulates STIM2 function, binding to two distinct sites on STIM2. So, again, a potentially intriguing and complex mechanism for the regulation of a second system, STIM2, by RGS10 and CaM. The main goal of this project was to uncover and characterize the physical details of the interaction of RGS10 and CaM. We recombinantly produced and purified full-length RGS10 and the RGS domain of RGS10 (residues S31-L165), as well as two mutants. By monitoring the intrinsic tryptophan fluorescence of these RGS10 proteins, we measured their affinities for CaM. The different RGS10 constructs bind to CaM with a low micromolar affinity, which is Ca2+ dependent. Using NMR spectroscopy, we confirmed this interaction and identified the residues of RGS10 that interact with CaM, based on NMR chemical shift changes. Using the individual C-terminal (M76-K148) and N-terminal (A1-D80) domains of CaM, we revealed that RGS10 binds preferentially to the C-terminal domain of CaM. Isothermal titration calorimetry measurements indicate that this interaction is endothermic and entropically driven. We also produced a soluble construct of one of the domains of STIM2 known to bind to CaM and suspected to bind to RGS10. We confirmed CaM binding to this STIM2 domain, but due to an unusual experimental limitation we were unable to perform the experiment to demonstrate the direct interaction of STIM2 and RGS10. Overall, our results represent the critical first steps towards a comprehensive understanding of calmodulin regulation of G protein signaling.