A set S of vertices is a determining set for a graph G if every automorphism of G is uniquely determined by its action on S. The determining number of G, denoted Det(G), is the size of a smallest determining set. This paper begins by proving that ifm is the prime factor decomposition of a connected graph then Det(G) = max{Det(G k i i )}. It then provides upper and lower bounds for the determining number of a Cartesian power of a prime connected graph. Further, this paper shows that Det(Q n ) = log 2 n +1 which matches the lower bound, and that Det(K n 3 ) = log 3 (2n+1) +1 which for all n is within one of the upper bound. The paper concludes by proving that if H is prime and connected, Det(H n ) = (log n).᭧