Multi-threshold CMOS (MTCMOS) is the most widely used circuit technique for suppressing subthreshold leakage currents in idle circuits. When a conventional MTCMOS circuit transitions from SLEEP mode to ACTIVE mode, voltages of power and ground distribution networks are disturbed. Mode transition noise phenomenon in MTCMOS circuits is examined in this chapter. An MTCMOS circuit technique with three operating modes (tri-mode) is described for noise suppression during activation events. A threshold voltage tuning methodology is presented to further alleviate the mode transition noise with smaller sleep transistors in MTCMOS circuits. Alternative applications of tri-mode MTCMOS for data preservation and leakage power reduction in idle memory elements are also discussed.