Operation of complementary metal-oxide-semiconductor (CMOS) logic gates at low supply voltages down to 100mV and ring oscillators down to 67mV is experimentally investigated. The measured voltage transfer characteristics of CMOS inverters and logic gates are explained using the subthreshold operation of MOS transistors. Robust control of ring oscillators allows for a tuning range of six decades in frequency with excellent sensitivity as low as 75mV per frequency decade at low voltages in the range from 0.2 to 0.5V, when body biasing of the metal-oxide-semiconductor field-effect transistors (MOSFETs) is also used. For the present state-of-the-art CMOS, the simple digital CMOS cells can operate properly at supply voltages down to 0.2V, but to achieve operation at lower voltages, additional circuitry is needed to maintain the matching between p- and n-MOSFETs, and the circuit complexity increases. Nevertheless, the experiments with ring oscillators demonstrated good performance of CMOS circuits down to the physical limit of two to three times the thermal voltage, while logic gates with stacked transistors need a supply of approximately four to six times the thermal voltage.