RFID tag tracking according to embodiments of the invention uses apparatus and methods for tracking backscatter RFID tags using the phase and receive signal strength of the tag signal. The tag tracking in some embodiments is accomplished with a computationally efficient recursive procedure to update a tag state estimate on each new response of the tag based on the previous tag state estimate and the measured phase of the tag signal. Some embodiments use a Monte Carlo simulation based on the previous tracking algorithm state and a statistical model of the forces acting on the tag. A system according to example embodiments of the invention can include a processor connected to a quadrature mixer. The processor is operable, for example through the use of firmware or software, to estimate a tag state of an RFID tag.