Proteasome inhibitors are potent inducers of cell death. The cytotoxic effect of proteasome inhibitors in general appears to be selective for proliferating cells, while quiescent cells seem to be protected. Conflicting results have been reported on the role of the CKI p27(Kip1) either in promoting or inhibiting apoptosis mediated by proteasome inhibitors and other drugs. Here I discuss the role of p27(Kip1) in apoptosis and chemotherapy of cancer.