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We present a novel temperature/leakage sensor, developed for high-speed, low-power, monitoring of processors and complex VLSI chips. The innovative idea is the use of 4T SRAM cells to measure on-chip temperature and leakage. Using the dependence of leakage currents to temperature, we measure varying decay (discharge) times of the 4T cell at different temperatures. Thus, decaying 4T sensors provide a digital pulse whose frequency depends on temperature. Because of the sensors' very small size, we can easily embed them in many structures thus obtaining both redundancy and a fine-grain thermal picture of the chip. A significant advantage of our sensor design is that it is insensitive to process variations at high temperatures. It is also relatively robust to noise. We propose mechanisms to measure temperature that exploit the sensor's small size and speed to increase measurement reliability. Decaying 4T sensors also provide a measurement of the level of leakage at their sensing area, allowing us to adjust leakage-control policies. Our 4T sensors are significantly smaller, faster, more reliable, and power efficient compared to the best previously proposed designs enabling new approaches to architectural-level thermal and leakage management.