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. As this book covers the basic aspects of the applications of cation–π interactions it is useful for students and researchers in a wide variety of scientific fields. In addition, the book will not only facilitate understanding of the phenomena occurring in various systems but also provides insights into the design of new catalysts, reactions, and materials.