In this chapter, we describe a new type of paper substrate based on cellulose nanofibers for flexible electronic applications. Cellulose nanofiber paper, referred to as nanopaper in this study, has high optical transparency like that of polyethylene terephthalate (PET) films and a low coefficient of thermal expansion comparable to that of glass. These excellent properties mean that cellulose nanopaper is expected to offer a promising alternative to glass and plastics and can be used as a transparent flexible substrate for a wide array of electronic devices. We also discuss transparent flexible electrodes based on cellulose nanopaper. The uniform coating of conductive materials such as silver nanowires and carbon nanotubes is accomplished by the simple filtration of their aqueous dispersions through cellulose nanopaper. The paper is used as a filter and a transparent flexible substrate. As-prepared conductive networks on nanopaper give superior transparent conductive performance and flexibility compared with those on PET films prepared by conventional coating processes. These findings open new doors for future flexible paper electronics.