This chapter introduces polymers with high heat resistance. The heat resistance of polymeric materials is classified into physical heat resistance, which is indexed by the glass transition temperature (Tg) and/or melting temperature (Tm), and chemical heat resistance, which is indexed by the temperature at which the polymer chemically decomposes (Td), and here the polymers with both high physical and chemical heat resistance are addressed. At first, general principles of molecular design to improve physical and chemical heat resistance of polymers are explained. Then, highly heat-resistant thermoplastic polymers with repeating structures based on such molecular design, specifically engineering plastics such as wholly aromatic polyamides, polyimides, polybenzoxazoles, poly(p-phenylene sulfide), polysulfone, poly(ether sulfone), and liquid crystal polyesters, are described. Syndiotactic polystyrene and fluoropolymers such as poly(tetrafluoroethylene) are also introduced as high heat-resistant polymers synthesized by polymerization of vinyl monomers. Furthermore, as another class of polymeric materials with high heat resistance, thermosetting resins that give highly cross-linked network polymers after curing are described. Initially, the general characteristics of thermosetting resins are reviewed, and then individual resins such as phenolic resins, benzoxazine resins, epoxy resins, cyanate ester resins, and bismaleimide resins are described in terms of their synthesis methods, curing reactions, properties of the cured products, applications, and so on.