In this chapter, electrochemical double layer capacitors (EDLCs) and pseudo-capacitors, both named as supercapacitors, are introduced. Recent research trends and applications are presented to help readers understand the performance and limitations of the state-of-the-art supercapacitors, with an emphasis on the role of hierarchical nanostructures in high-performance electrodes. The working principles of supercapacitors are overviewed, and the electrochemical performance of various nanomaterials such as carbon nanotubes, graphene, metal oxide nanoparticles, and conductive polymer nanowires is discussed. The hierarchical nanostructures of these nanomaterials can enable active control of porosity and realize hybrid electrode systems benefitting from multi-functionality of the constituent materials. Various combinations of nanomaterials for building hierarchical nanostructures are reviewed, and the technical merits of these hybrid systems are discussed.