Introduction In a recent article MacKinnon [I] describes four methods that may be used to find square roots of 2 x 2 matrices. The first of these methods requires that the matrix for which the square roots are sought be diagonalizable and, subsequently, this method was used by Scott 121 to determine all the square roots of 2 X 2 matrices. A surprising conclusion is that scalar 2 x 2 matrices possess double-infinities of square roots whereas nonscalar 2 x 2 matrices have only a finite number of square roots. The purpose of this article is to show how the Cayley-Hamilton theorem may be used to determine explicit fonnulae for all the square roots of 2 X 2 matrices. These formulae indicate exactly when a 2 x 2 matrix has square roots, and the ilumber of such roots. By definition, the square roots of a 2 X 2 matrix, A, are those 2 X 2 matrices, X, for which X" A. However, for each square matrix X, the Cayley-Hamilton theorem states that