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 (tr X)X + (det X)I = 0. X" (1