This book introduces the principles and techniques of modern electron paramagnetic resonance (EPR) spectroscopy that are essential 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 (also 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 examples of semiconductors and insulators. While the non-specialist learns about the potential of the different methods, the researcher finds help in the application of commercial apparatus and guidance from ab initio theory for deriving structure models from data.