This chapter discusses flexible functional materials that, on the one hand, allow one to eliminate some of the disadvantages of paper, and on the other hand, give paper the necessary functionality. Their flexibility and stretchability allow them to be easily manipulated or bent while maintaining their functionality. Depending on the tasks to be solved, flexible functional materials can be organic or inorganic semiconductors, metals or dielectrics, have increased electronic or ionic conductivity, be catalytically active, exhibit high gas sensitivity, etc. Various composites based on polymers, metal and metal oxide nanoparticles, graphene, carbon nanotubes, 2D nanomaterials, and quantum dots can also be used in the manufacture of paper-based devices. It is shown that the use of functional materials allows the development of a wide variety of paper-based sensors and devices from humidity sensors and field-effect transistors to energy technology devices, such as solar cells.