A variety of adhesives are used depending on the properties and mechanical strength requirements of products assembled with various composite materials in our casual life and many industrial fields. In this chapter, the author refers to the mechanism of adhesion and the characteristics of adhesive polymers, and some points to obtain enough adhesion strength, toughness, and reliability. A large number of polymers used for adhesive types are classified based on their origin and material characteristics, as well as several matters related to adhesives and adhesion. In the following sections, the features of adhesives are described from the viewpoints of network formation, high strength, and toughness of the polymer materials. By designing the polymer network structure and controlling the reactions during the curing process, the thermosetting resins exhibit excellent heat resistance and mechanical strength, being valid for use in adhesion systems. The author also describes the recent developments of several functional adhesives, such as stimuli-responsive adhesives, semiconductor manufacturing-related adhesives, dismantlable adhesion, and dissimilar materials adhesion systems. The final part of this chapter presents several case studies on dismantlable adhesion and dissimilar materials bonding.