Tremendous technological developments and rapid progress in theory have opened a new area of modern physics: high-field electrodynamics, characterized by two fundamental scales: the relativistic intensity regime and the quantum electrodynamics (QED) scale. This text focuses on the theory of electromagnetic interactions near the relativistic intensity regime. It gives a comprehensive, detailed description of the theoretical tools needed to approach the subject and serves as a starting point for more advanced and specialized research. Written at a somewhat advanced level, the book includes many topics that have not been previously discussed within the context of high-field electrodynamics.