An apparatus for detecting foreign object debris, the apparatus comprising: at least two detectors arranged in an array, wherein each detector is configured so that when the detector is positioned between a power source and a power receiver of a wireless power transfer system, the detector is configured to generate an electrical signal based on a magnetic field between the power source and the power receiver; and control electronics coupled to the detectors, wherein the control electronics are configured to: measure the electrical signals of the detectors in response to a magnetic field between the power source and the power receiver; and determine whether foreign object debris is positioned between the power source and the power receiver by comparing baseline information to the measured electrical signals, wherein the baseline information comprises information about the electrical signals generated by the detectors when no foreign object debris is positioned between the power source and the power receiver; and characterized in that: the control electronics are configured to compare the baseline information to the measured signals by determining mean and covariance matrices for the baseline information and determining whether foreign object debris is positioned between the power source and the power receiver based on the mean and covariance matrices. at least two detectors arranged in an array, wherein each detector is configured so that when the detector is positioned between a power source and a power receiver of a wireless power transfer system, the detector is configured to generate an electrical signal based on a magnetic field between the power source and the power receiver; and control electronics coupled to the detectors, wherein the control electronics are configured to: measure the electrical signals of the detectors in response to a magnetic field between the power source and the power receiver; and determine whether foreign object debris is positioned between the power source and the power receiver by comparing baseline information to the measured electrical signals, wherein the baseline information comprises information about the electrical signals generated by the detectors when no foreign object debris is positioned between the power source and the power receiver; and characterized in that: the control electronics are configured to compare the baseline information to the measured signals by determining mean and covariance matrices for the baseline information and determining whether foreign object debris is positioned between the power source and the power receiver based on the mean and covariance matrices. measure the electrical signals of the detectors in response to a magnetic field between the power source and the power receiver; and determine whether foreign object debris is positioned between the power source and the power receiver by comparing baseline information to the measured electrical signals, wherein the baseline information comprises information about the electrical signals generated by the detectors when no foreign object debris is positioned between the power source and the power receiver; and characterized in that: the control electronics are configured to compare the baseline information to the measured signals by determining mean and covariance matrices for the baseline information and determining whether foreign object debris is positioned between the power source and the power receiver based on the mean and covariance matrices.