The impedance spectra for aluminum alloys and aluminum-base metal matrix composites have been found to change drastically when localized corrosion occurred during exposure to 0.5N NaCl. Similar results have been obtained for a model pit in aluminum 6061 alloy. These changes include a large increase of the capacitance and the occurrence of a transmission line element. A model for the impedance behavior has been developed which can be used for the analysis of the experimental data. Under certain assumptions it is possible to obtain pits growth rates and their changes with exposure time from measurements of the polarization resistance of the pit Rpit at the corrosion potential. Electrochemical impedance spectroscopy (EIS) has been found to be a very sensitive tool for detecting the initiation and propagation of localized corrosion of aluminum alloys. Examples are given for Al 6061-T6, Al 7075-T6, aluminum/silicon carbide and aluminum/graphite during exposure to NaCl.