In this paper a novel completion detection method for self-timed, asynchronous subthreshold circuits is presented. By employing the self-timed operation principle, substantial speed gains in the operation of the asynchronous pipelines can be realized. The completion detection system is very simple, consisting of a sensor transistor, a very basic AC-coupled amplifier and a monostable multivibrator. The proposed method can be easily integrated into the CMOS design flow. The advantages of the proposed completion detection system is shown through simulations on an 16-bit ripple carry adder in a standard 0.18 mum CMOS process operating at 400 mV supply voltage.