Electrochemical impedance spectroscopy (EIS) based sensor technology is used for the nondestructive evaluation of the bond between external carbon fiber reinforced polymer (CFRP) reinforcement and concrete in beams. Copper tape on the surface of the CFRP sheet, stainless steel wire embedded in the concrete, and reinforcing bars were used as the sensing elements. Laboratory experiments were designed to test the capability of the sensors to detect the debonding of the CFRP from the concrete and to study the effect of short-term (humidity and temperature fluctuations) and long-term (freeze-thaw and wet-dry exposure, and rebar corrosion) environmental conditions on the measurements. The CFRP sheet was debonded from the concrete and impedance measurements were taken between various pairs of electrodes at various interfacial crack lengths. The dependence of the impedance spectra, and of the parameters obtained from equivalent circuit analysis, on the interfacial crack length was studied. Capacitance parameters in the equivalent circuit correlated strongly with the interfacial crack length and can be used to assess the global state of the bond between CFRP reinforcement and concrete. Impedance measurements taken between embedded wire sensors can be used to detect the location of debonded regions. Although the measurements are sensitive to short- and long-term environmental effects, measurements at high frequencies and the capacitance parameters resulting from equivalent circuit analysis are relatively insensitive to these factors and are effective indicators of debonding.