The cation-π interaction is an attractive noncovalent interaction between a cation and a π system. Due to the stronger interaction energy than those of the other π interactions, such as π-π and CH-π interactions, the cation-π interaction has recently been recognized as a new tool for controlling the regio- and stereoselectivities in various types of organic reactions. This review attempts to cover a variety of organic reactions controlled by cation-π interactions, which includes not only recent examples but also those reported before the term "cation-π interaction" was defined in 1990. This review will provide comprehensive knowledge on the role of cation-π interactions in organic synthesis.