Thanks to its unique ability to deliver information about the structure, spatial distribution, and even dynamics of paramagnetic species, electron spin resonance (ESR) spectroscopy is one of the most powerful analytical techniques in modern life sciences. Recently, the method has gained significant attention as a tool to study oxidative stress, i.e., a state in which the number of reactive oxygen species (ROS) exceeds the physiological antioxidative potential of the cells, which plays an important role in the development of many chronic diseases as well as acute stress conditions.