Film capacitors with high energy storage are becoming particularly important with the development of advanced electronic and electrical power systems. Polymer-based materials have stood out from other materials and have become the main dielectrics in film capacitors because of their flexibility, cost-effectiveness, and tailorable functional properties. A core issue for achieving films with high performance is to process a material with high dielectric permittivity and breakdown strength but low dielectric loss. Nanocomposites and all-polymer dielectrics are two promising avenues to address this issue. Thus, in this work, the advances of the past decade are highlighted to reflect the latest developments in polymer-based film dielectrics. The key factors of dielectric properties, including polarization mechanisms, dielectric breakdown strength, filler-matrix interface, and hierarchical structures are discussed in detail. A comprehensive understanding of the fabrication of polymer-based materials with special constituents, molecular segments, and microstructures will provide guidance to engineers on the fabrication of polymer-based materials with high performance.