Let m be a given positive integer and let A be an m × m complex matrix. We prove that the discrete systemis Hyers-Ulam stable if and only if the matrix A possesses a discrete dichotomy. Also we prove that the scalar difference equation of order m x n+m = a 1 x n+m−1 + a 2 x n+m−2 + · · · + a m x n , n ∈ Z + , is Hyers-Ulam stable if and only if the algebraic equation z m = a 1 z m−1 + · · · + a m−1 z + a m , z ∈ C has no roots on the unit circle. This latter result is essentially known, for further details see for example [24] and [2]. However, our proofs are completely different and moreover, it seems that our approach opens the way to obtain many other results in this topic.