This is a postgraduate book, partly for electronics engineers, who would find a good deal of the material difficult, because it is a comprehensive account of the subject which demands a good deal of familiarity with quantum mechanics. The contents include chapters on internal structure and energy levels of atoms, the magnetic properties of atoms, energy levels of paramagnetic maser materials, paramagnetic resonance and the dynamics of spin systems, the three level solid state maser and travelling wave maser.