This book introduces the principles and techniques of modern electron paramagnetic resonance (EPR) spectroscopy that are essential for applications used to determine microscopic defect structures. Many different magnetic resonance methods are required for investigating the microscopic and electronic properties of solids and uncovering correlations between those properties. In addition to EPR such methods include electron nuclear double resonance (ENDOR), electronically and optically detected EPR (the latter is known as ODENDOR), and electronically and optically detected ENDOR. This book comprehensively discusses experimental, technological, and theoretical aspects of these techniques from a practical point of view with many illustrative examples taken from semiconductors and insulators. The non-specialist is informed about the potential of the different methods. A researcher finds practical help in the application of commercial apparatus as well as useful guidance from ab initio theory for the task of deriving structure models from experimental data.