Polymer dielectrics-based capacitors are indispensable to the development of increasingly complex, miniaturized and sustainable electronics and electrical systems. However, the current polymer dielectrics are limited by their relatively low discharged energy density, efficiency and poor high-temperature performance. Here, we review the recent advances in the development of high-performance polymer and composite dielectrics for capacitive energy storage applications at both ambient and elevated temperature (≥ 150 °C). We highlight the underlying rationale behind the material development by constructing the relationship between the physical properties of materials and their energy-storage capability. Challenges and future opportunities are discussed at the end of the review.