The present disclosure is directed to determining speeds of radio frequency tags. In some implementations, a method includes sampling a Radio Frequency (RF) signal from an RFID tag at a plurality of different times as the RFID tag approaches an RFID reader and recedes from the RFID reader. A phase of the RF signal is determined for each of the plurality of different times based, at least in part, on the sampled RF signal. A speed of the RFID tag is determined based, at least in part, on the determined phase for the sampled signal for each of the plurality of different times.