For many proteins, flexibility and motion form the basis of their function. In our lab, we quantify the conformational landscapes of proteins and their changes upon interaction with ligands and other proteins. We use Double Electron-Electron Resonance (DEER) spectroscopy, a form of Electron Paramagnetic Resonance (EPR) spectroscopy. Using DEER, we directly measure absolute distances and distance distributions within proteins. From the data, we build quantitative structural models of the protein's intrinsic flexibility, conformational substates, and the structural changes induced by ligands and binding partners. We are also active in the development of advanced DEER methods. In this presentation, we present our work on ion channels involved in the regulation of the heartbeat and their bacterial homologs. Their activity is modulated by the binding of cyclic nucleotides. The structural mechanism underlying this modulation is unknown. We show how DEER enabled us to gain insight into the structure, energetics, and kinetics of these membrane proteins.