In the quest of operating a circuit at its minimum energy, both of the design-time and run-time optimizations are being employed aggressively in mobile devices today. This paper overviews some of the energy reduction techniques and highlights the role of device-circuit interaction in achieving minimum energy operation. We show that run-time energy minimization can be achieved by keeping the ratio between static energy and dynamic energy to an optimum value. Focusing on the ease-of-design, we then review several delay-based sensor circuits to monitor transistor performance, leakage-current and temperature. We also discuss activity monitors to estimate the dynamic energy. We then discuss a cell-based design approach for both of the sensing and the back-gate voltage tuning circuits for area and cost reduction. Using the sensing and the voltage tuning circuits, the system can operate at its minimum energy all the time.