Thermal management is an important design issue in high performance, low-power portable computers. ofthe computer system is designed for worst-case processor power dissipation and environmental operating conditions, it will carry an area and cost penalty for the system designer: The next generation PowerPCTM microprocessor includes a Thermal Assist Unit (TAU) comprised of an on-chip thermal sensor and associated logic. The TAU monitors the junction temperature of the processor and dynamically adjusts processor operation to provide maximum performance under changing environmental conditions. The TAU is used in conjunction with other low power features such as dynamic power management, instruction cache throttling, and static low power tnodes to provide comprehensive power and thermal management. This paper will describe the implementation of the TAU and present characterization and operating data from$rst silicon.