Time-Varying Fields in Crystals: Creating Coherence with Light explores the physics of coherent, time-dependent phenomena in solids. Bridging condensed-matter physics, optics, and ultrafast science, it develops a unified framework for understanding the interaction of coherent radiation, spanning frequencies from the terahertz to the ultraviolet, with the vibrational and electronic degrees of freedom of crystals.Beginning with the fundamental concepts of coherence in solids, the book examines how light pulses generate coherent excitations and their associated time-varying fields, and how these evolve, interact, and modify material properties. Particular attention is given to the ultrafast generation and control of coherent phonons, magnons, superconducting excitations, excitons, and polaritons, as well as to nonlinear phononics. Few-level systems are also considered, illustrating how, under appropriate conditions, excitations in crystals can exhibit dynamics normally associated with atoms and molecules.A central theme is the progression from spectroscopy to control. Rather than treating radiation solely as a means of probing equilibrium properties, the book emphasizes its ability to prepare and drive matter into nonequilibrium states with well-defined phase relationships. Extensive experimental examples illuminate the underlying physics and connect theoretical descriptions with modern ultrafast methods employing terahertz, optical, and x-ray pulses.Written for graduate students and researchers in condensed-matter physics, optical physics, materials science, and related fields, the book combines fundamental principles with contemporary developments. It offers a broad perspective on coherent light-matter interactions and the rapidly advancing ability to manipulate the dynamical properties of solids using electromagnetic pulses.