This book provides a foundational understanding of polymer dielectrics based on percolative composites. It covers the microstructure and physical properties, such as dielectric, electrical, magnetic, and rheological properties, of polymer composites, as well as how these properties can be explained using various theoretical models and spectroscopy techniques, such as dielectric spectroscopy, impedance spectroscopy, and conductivity spectroscopy. The book also discusses non-percolative polymer composites and the suitability of polymer dielectrics for electrical energy storage in various devices. It is intended for graduate students and professionals in fields such as condensed matter physics, applied physics, statistical physics, materials science, polymer science and technology, chemistry, and engineering. It will be particularly useful for physicists, materials scientists, polymer scientists, chemists, engineers, and others interested in the physics and applications of percolativecomposites based on polymer matrix.