The notion of simultaneous reduction of pairs of matrices and linear operators to triangular forms is introduced and a survey of known material on the subject is given. Further, some open problems are pointed out throughout the text. The paper is meant to be accessible to the non-specialist and does not contain any new results or proofs. 1. Background information. The well-known theorem of Schur (see for example [23]) states that if A is a complex m× m matrix, then there exists a unitary m× m matrix U , such that U −1 AU is an upper triangular matrix. In other words, each square complex matrix can be reduced to upper triangular form by a unitary similarity transformation. For pairs of square complex matrices, the following result was obtained by McCoy [24]. Theorem 1. Let A, Z be a pair of complex m × m matrices. Then the following two statements are equivalent : 1. There exists an invertible m × m matrix S, such that both S −1 AS and S −1 ZS are upper triangular matrices. 2. For each polynomial p(λ, µ) in the non-commuting variables λ and µ, the m × m matrix p(A, Z)(AZ − ZA) is nilpotent. A pair of m × m matrices A, Z which satisfies the statements of Theorem 1 is said to admit simultaneous reduction to upper triangular form. The proof of Theorem 1 in [24] is rather involved. Elementary proofs of this theorem have been obtained in [13] and [17]. The theorem is made more explicit for certain pairs of matrices in [19] and [20]. Further, a recent extension of Theorem 1 is given in [26]. The literature on this subject, which includes a paper of Frobenius [16] of almost a century ago, is extensive. For more information and references, see [21]. Generalizations of Theorem 1 to an infinite dimensional context has been obtained in [22] and [25]; see also Section 3.