A surprisingly large number of materials have unpaired electrons. These include free radicals, many transition metal ions, and defects in materials. Free radicals are often short-lived, but play crucial roles in many significant processes such as photosynthesis, oxidation, catalysis, and polymerization reactions.
Electron paramagnetic resonance (EPR) spectroscopy is the only method for the direct detection of species that have unpaired electrons, and its applications span one of the widest ranges of any analytical technique.