Polymer–polymer interfaces play an important role in the properties of polymer blends. This chapter discusses the theoretical aspects of interfaces, reinforcement of interfaces by copolymers, and interfacial reaction of polymers. It introduces theories of polymer–polymer interfaces that are often used to analyze the interface of polymers. The chapter presents the details of the experimental findings concerning the segregation, diffusion, and adhesion of block copolymers. Based on the understanding of the segregation of block copolymers, it also presents what is known about the adhesion aspect of polymer–polymer interfaces reinforced by block copolymers. The chapter also discusses the reaction of functional polymers at interfaces, focusing on the analogy and difference between the interfacial reaction of end-functionalized polymers and the segregation of premade block copolymers. A classical example of interfacial reinforcement is the use of block copolymers as compatibilizers. For industrial applications, however, a variety of copolymers other than block copolymers are used as compatibilizers.