Foreign object detection (FOD) is provided for wireless energy transfer systems. A transmitter receives an input voltage, converts it to an output current, supplies the output current to a transmit coil, and samples a first set of analog signals to generate a first set of digital values. A receiver converts a current induced in a receive coil by energy transferred from the transmit coil into an output voltage, samples a second set of analog signals to generate a second set of digital values, and communicates the second set of digital values to the transmitter. The transmitter generates a FOD signal based upon the first and second sets of digital values that indicates detection or non-detection of a foreign object within the energy transfer. In further embodiments, a comparison of the detected power loss or efficiency to expected power loss or efficiency is used to generate the FOD signal.