This chapter discusses our research into the functionalization of polyolefins (PE, PP etc.) that contain polar groups (such as OH and NH 2) in the side chains and chain end, as well as polyolefin graft and block copolymers containing both a polyolefin block and functional polymer blocks (acrylic and methacrylate polymers). In the late 1980s, our research on the functionalization of polyolefins was inspired by the development of homogeneous, single-site metallocene catalysts that showed excellent copolymerization capabilities and a well-controlled polymerization mechanism. We were curious to know how to apply this newly available technology to the direct polymerization (in-reactor) process to circumvent the inevitable deactivation of the transition metal cationic active site by functional (polar) groups containing basic O, N, and halides. Throughout the past two decades, we have developed an effective approach that is centered on specially designed "reactive" comonomers and chain transfer agents. These can deliver three essential properties during the polymerization process: (1) stability (no side reaction) of the active site; (2) solubility in the polymerization media for effective incorporation; and (3) versatility for interconversion to desirable functional groups under mild reaction conditions after polymerization (preferably a one-pot process). With the conjunction of suitable metallocene catalysts, a broad range of polyolefins with "reactive" sites in the side chains or chain end were prepared, and the incorporated reactive groups were interconverted into functional groups to form side-chain-or chain-end-functionalized polyolefins. Some active sites were also transformed into "living" radical or anionic initiator to initiate graft-from polymerization of polar monomers to obtain polyolefin graft and block copolymers (such as PP-g-PMMA and PP-b-PMMA). The resulting functional polyolefins show many desirable properties and applications. This chapter focuses on applications in the area of energy storage. Specifically, I will discuss the usage of new functional polypropylene polymers in the formation of dielectric thin films in capacitors that show significantly higher energy and power densities.