The interlaminar interface (i.e., the interface between laminae) of continuous carbon fiber polymer-matrix structural composites was monitored in real time during dynamic changes in temperature, humidity and stress by measurement of the contact electrical resistivity of the interface. The stress was compressive, in the direction perpendicular to the interlaminar interface. Temperature, humidity and stress were all found to have reversible effects on the resistivity, due to the effect of temperature on the probability of the jump of an electron from one lamina to the adjacent one, and the effects of humidity and stress on the extent of contact between fibers of adjacent laminae. In addition, due to damage, temperature caused the resistivity to increase whereas stress caused the resistivity to decrease.