Comments on higher derivative operators in some SUSY field theories

We study the leading irrelevant operators along the flat directions of certain supersymmetric theories. In particular, we focus on finite N = 2 (including N = 4) supersymmetric field theories in four dimensions and show that these operators are completely determined by the symmetries of the problem. This shows that they are generated only at one loop and are not renormalized beyond this order. An instanton computation in similar three dimensional theories shows that these terms are renormalized. Hence, the four dimensional non-renormalization theorem of these terms is not valid in three dimensions.