Capacitive sensors with various electrode configurations have been developed for materials evaluation and defect/damage detection of low-conductivity structures. Complex permittivity is an important material property that characterizes the electrical behavior of low-conductivity structures. One effective and low-cost approach to obtain information on material complex permittivity is through capacitance measurements. In the current research, numerical models have been developed to relate the complex permittivity of planar and cylindrical test-pieces to the capacitive sensor response. In other words, the existence of defects (change in material complex permittivity) can be deduced from changes in the sensor capacitance. A handheld probe has also been developed for practical usage.