A popular pulsed EPR experiments currently is double electron-electron resonance (DEER), which is also known as pulsed electron-electron double resonance (PELDOR). In this experiment, two frequencies control two spins to probe their coupling. The distance between the spins can then be inferred from their coupling strength. This information is used to elucidate structures of large bio-molecules. PELDOR spectroscopy is a versatile tool for structural investigations of proteins, even in a cellular environment.