Spin states created far from the thermal equilibrium, i.e. non-Boltzmann states, such as radical pairs and triplet states, can be used as a sensible probe for investigating the structure-function relationship of both, biological molecules as well as organic semiconductors. The prime method to directly observe these hyperpolarised electron spin states is time-resolved electron paramagnetic resonance (TREPR) spectroscopy. This is a pump–probe experiment with the electron spin as probe and the EPR spectrometer as readout, correspondingly. Here we show a series of applications from both, biology and materials science.