Electron paramagnetic resonance (EPR) is a spectroscopic tool for the study of materials containing unpaired electron spins (e.g. organic radicals, free charge carriers, semiconductor defect states or transition metal ions). In our lab EPR is employed to study structure-function relationships of paramagnetic states in materials relevant for energy production and conversion, in particular photovoltaic (PV) devices. For this purpose dedicated multi-frequency and multi-resonance EPR instrumentation from GHz to THz frequencies is available in our lab. For in operando studies of spin-dependent charge transport processes in fully processed devices we design and employ novel indirect (optical and/or electrical) detection schemes.