Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In the mathematical field of representation theory, a reductive dual pair is a pair of subgroups (G,G ) of the isometry group Sp(W) of a symplectic vector space W, such that G is the centralizer of G in Sp(W) and vice versa, and these groups act reductively on W. Somewhat more loosely, one speaks of a dual pair whenever two groups are the mutual centralizers in a larger group, which is frequently a general linear group. The concept was introduced by Roger Howe in an influential preprint of the 1970s, which was ultimately published as Howe (1989a).