Methods of Experimental Physics, Volume 3, Part B: Molecular Physics, Second Edition presents the basic principles of electron spin resonance spectrometers and the electron spin resonance spectroscopy. This four-chapter text addresses the concept of thermal equilibrium and relaxation. Some of the topics covered in the book are the features of nuclear quadrupole resonance spectrometers; basic principles of radio-frequency spectrometers; computer methods in magnetic resonance; components of electron spin resonance spectrometer systems; and the resonance condition. Other chapters deal with the ionization potential of free radicals, the electron affinities by mass spectrometric methods, and the experimental methods for determining appearance potentials. The discussion then shifts to the theory of the incoherent neutron scattering and its application to molecular dynamics. The final chapter is devoted to the spectrometer operation and design. The book can provide useful information to chemists, physicists, students, and researchers.