As technology scales, subthreshold leakage currents grow exponentially and become an increasingly large component of total power dissipation. CAD tools to help model and manage subthreshold leakage currents will be needed for developing ultra low power and high performance integrated circuits. This paper gives an overview of current research to control leakage currents, with an emphasis on areas where CAD improvements will be needed. The first part of the paper explores techniques to model subthreshold leakage currents at the device, circuit, and system levels. Next, circuit techniques such as source biasing, dual Vt partitioning, MTCMOS, and VTCMOS are described. These techniques reduce leakage currents during standby states and minimize power consumption. This paper also explores ways to reduce total active power by limiting leakage currents and optimally trading off between dynamic and leakage power components.