The demand for smaller and lighter capacitors with higher temperature capability is increasing in the power electronic industry and military arena. This article reviews existing capacitor technologies and the potential new capacitor technologies toward realizing these goals. Various dielectric materials beneficial to high dielectric constant and breakdown strength potentially meeting the high requirements are discussed. The importance of dielectric composites including selection of nanofillers and matrix is discussed. The successful implementation of these dielectric materials for high energy density purpose requires a detailed understanding of both materials engineering and processing.