In this paper, the classical CMOS Schmitt trigger (ST) operating in the subthreshold regime is analyzed. The complete DC voltage transfer characteristic of the CMOS ST is determined. The metastable segment of the characteristic is explained in terms of the negative resistance of the NMOS and PMOS subcircuits of the ST. Small-signal analysis is carried out to determine the minimum supply voltage at which the hysteresis appears and to obtain a rough estimation of the hysteresis width. It is shown that the theoretical minimum supply voltage required to obtain hysteresis is 2ln(2 + √5)kT/q = 75 mV at room temperature. A test chip with CMOS Schmitt triggers was designed and fabricated in a 180 nm technology in order to study their operation at supply voltages between 50 mV and 1000 mV.