This paper presents an analysis of impedance matching in passive radio frequency identification (RFID) tags. RFID chip impedance varies with frequency and with the power absorbed by the chip. The variation of this impedance with power can drastically affect the performance of the chip. We show how the impedance variation of the chip front makes the tag non-operational at certain power levels or can create a "dead-zone" where the tag is non-responsive. Impedance matching must also take into account the actual application of the tag. We present guidelines for impedance matching and show that conjugate matching is not the best choice for many applications.