This book introduces the role of cation–π interactions in chemistry, biology, and polymer materials science. Since the cation–π interaction was defined in 1990, its importance has been recognized in a variety of fields due to its relatively significant interaction energy. A number of examples are introduced in which the contribution of cation-π interactions was evidenced by X-ray structures, spectroscopies, computations, and other means. The latest developments in these areas are remarkable; therefore, many recent examples are included.