Wide utilization of portable battery-operated devices in modern applications triggers a demand for ultra low-power designs. Many circuit techniques have been successfully applied to reduce both dynamic and leakage components of power. Recently, digital subthreshold circuit design has become a very promising method for ultra-low power applications. Circuits operating in the subthreshold region utilize a supply voltage (VDD) that is close to or even less than the threshold voltages of the transistors. This low V DD operation results in ultra low-power dissipation of the circuit, but significantly increases the circuit's propagation delay. In this paper a review of the advantages of the subthreshold circuits compared to the conventional strong inversion circuits is presented. Design challenges in advanced sub-micron technologies are presented through simulation results in 90nm and 65nm processes.