Because most stable molecules have a closed-shell configuration without a suitable unpaired spin, the technique is less widely used than NMR.
The degeneracy of the spin states is lifted by the presence of a strong magnetic field, and radiation in the microwave part of the electromagnetic spectrum is used to induce transitions between spin states.
To detect some subtle details of some systems, high-field-high-frequency electron spin resonance spectroscopy is required. While ESR is affordable for a medium-sized academic laboratory, there are few scientific centers in the world offering high-field-high-frequency electron spin resonance spectroscopy, among them ILL in Grenoble, France and one in Tallahassee, USA.