The J/Psi is a promising probe to study the hot and dense matter created in relativistic heavy ion collisions. Most theoretical models indicate that producing a quark-gluon plasma will result in changes to the J/Psi yield. PHENIX measured the J/Psi yield in Cu+Cu and Au+Au collisions. Preliminary results on the nuclear modification factor will be discussed as a function of the transverse momentum, rapidity and centrality and compared to expectations from various theoretical models.