Cross-linked monolithic polymer networks (or dried gels) with hierarchically porous structures have been prepared by controlled/living radical polymerization techniques such as nitroxide-mediated radical polymerization (NMP), atom transfer radical polymerization (ATRP), and organotellurium-mediated living radical polymerization (TERP). Well-controllable macropores are formed as a transient structure of spinodal decomposition induced during the polymerization of monomers, and smaller pores in the macropore skeletons are resulted from secondary phase separation during the phase separation–gelation process. Also, the obtained networks are pyrolyzed in an inert atmosphere to obtain carbonaceous materials with abundant micropores while preserving the macro- and mesoporous morphology. In addition, applications of such hierarchically porous monolithic materials to separation media and electrochemical devices are demonstrated. In the present article, the concept, mechanism, methodology, and experimental results are discussed in detail.