Fullerene derivatives exhibit many interesting structural, electronic, and chemical properties, largely determined by the carbon cage's ability to accept electrons. Electron spin resonance spectroscopy is a very sensitive and important technique to detect open shell species and to determine HOMO and LUMO density distributions. Here we present a short overview of the most significant and informative reports of electron spin resonance studies of empty and endohedral fullerenes, classified on the basis of the entrapped moiety, cage size symmetry, and on the techniques used to generate the paramagnetic states. Copyright © 2016 John Wiley & Sons, Ltd.