Book excerpt: From chemistry to solid state physics to biology, the applications of Electron Paramagnetic Resonance (EPR) are relevant to many areas. This unified treatment is based on the spin Hamiltonian approach and makes extensive use of irreducible tensor techniques to analyze systems in which two or more spins are magnetically coupled. This edition contains a new Introduction by coauthor Dante Gatteschi, a pioneer and scholar of molecular magnetism. The first two chapters review the foundations of exchange interactions, followed by examinations of the spectra of pairs and clusters, relaxation in oligonuclear species, approaches to infinite lattices, and how EPR can provide firsthand information on spin dynamics. Subsequent chapters explore experimental data, magnetically coupled systems, low-dimensional materials, and the use of EPR to characterize excitons and exciton motion. More than 200 figures and tables appear throughout the book, which concludes with a pair of helpful appendices.