Power gating is a technique commonly used for runtime leakage reduction in digital CMOS circuits. In microprocessors, power gating can be implemented by using sleep transistors to selectively deactivate circuit modules when they are idle during program execution. In this dissertation, a framework for power gating arithmetic functional units in embedded microprocessors with architecture and compiler support is proposed. During compile time, program regions are identified where one or more functional units are idle and sleep instructions are inserted into the code so that those units can be put to sleep during program execution. Subsequently, when their need is detected during the instruction decode stage, they are woken up with the help of hardware control signals. For a set of benchmarks from the MiBench suite, leakage energy savings of 27% and 31% are achieved (based on a 70 nm PTM model) in the functional units of a processor, modeled on the ARM architecture, with and without floating point units, respectively. Further, the impact of traditional performance-enhancing compiler optimizations on the amount of leakage savings obtained with this framework is studied through analysis and simulations. Based on the observations, a leakage-aware compilation flow is derived that improves the effectiveness of this framework. It is observed that, through the use of various compiler optimizations, an additional savings of around 15% and even up to 9X leakage energy savings in individual functional units is possible. Finally,in the context of multi-core processors supporting multithreading, three different microarchitectural techniques, for different multithreading schemes, are investigated for state-retentive power gating of register files. In an in-order core, when a thread gets blocked due to a memory stall, the corresponding register file can be placed in a low leakage state. When the memory stall gets resolved, the register file is activated so that it may be accessed again. The overhead due to wake-up latency is completely hidden in two of the schemes, while it is hidden for the most part in the third. Experimental results on multiprogrammed workloads comprised of SPEC 2000 integer benchmarks show that, in an 8-core processor executing 64 threads, the average leakage savings in the register files, modeled in FreePDK 45 nm MTCMOS technology, are 42% in coarse-grained multithreading, while they are between 7% and 8% in fine-grained and simultaneous multithreading. The contributions of this dissertation represent a significant advancement in the quest for reducing leakage energy consumption in microprocessors with minimal degradation in performance.