Features are investigated in eddy current data that are sensitive to corrosion and fatigue cracks in airframe structures while invariant to other NDE noise factors. To investigate subsurface corrosion characterization at the faying surface, a series of eddy current studies were performed using an analytical model for varying total subsurface thickness loss and percentage of the thickness loss occurring in each layer. Results for the simulated studies are presented demonstrating a novel feature for corrosion characterization using first and second order derivatives of the impedance response with respect to frequency. For characterization of subsurface cracks around fastener holes in structures, numerical simulations and experimental studies are presented. Unique features in the measurement response in circumferential direction were found to be sensitive to subsurface cracks around fastener holes and invariant to irregular geometric factors such as fastener fit and probe tilt. Multifrequency eddy current data combined with circumferential (spatial) measurement features were found to be promising for characterizing subsurface cracks in terms of length and depth.