Simultaneous multithreading (SMT) and chip multiprocessing (CMP) both allow a chip to achieve greater throughput, but their thermal properties are still poorly understood. This paper uses Turandot, PowerTimer, and HotSpot to evaluate the thermal eff iciency for a Power4/Power5-like core. Our results show that although SMT and CMP exhibit similar peak operating temperatures, the mechanism by which they heat up are quite different. More specifically, SMT heating is primarily caused by localized heating in certain key structures such as the register file, due to increased utilization. On the other hand, CMP heating is mainly caused by the global impact of increased energy output, due to the extra energy of an added core. Because of this difference in heat up machanism, we found that the best thermal management technique is also different for SMT and CMP. Finally, we show that CMP and SMT will scale differently as the contribution of leakage power grows, with CMP suffering from higher leakage due to the second core's higher temperature and the exponential temperature-dependence of subthreshold leakage.