The performance of subthreshold source-coupled logic (STSCL) circuits for ultra-low power applications is explored. It is shown that the power consumption of STSCL circuits can be reduced well below the subthreshold leakage current of static CMOS circuits. STSCL circuits exhibit a better power-delay performance compared to their static CMOS counterparts in situations where the leakage current constitutes a significant part of the power dissipation of static CMOS gates. The superior control on power consumption, in addition to lower sensitivity to the process and supply voltage variations make STSCL topology very suitable for implementing ultra-low-power low-frequency digital systems in modern nanometer scale technologies. An analytical approach for comparing the power-delay performance of these two topologies is proposed.