In this work, a complete passive UHF RFID system for capacitive sensing applications is developed to be used in food quality monitoring and supply chain management. All the components of the RFID system including the base station, the base station antenna, the tag chip, and the tag antenna is designed, implemented, and tested. The operation of all the components are shown by measurements and their performance is characterized. The remote powering subsystem consists of the tag antenna and the rectifier. Measurement results showed a maximum power conversion efficiency of 65% for the antenna-rectifier pair operated at 866 MHz. The rectifier is a three-stage, differential rectifier with threshold compensation, that offers high power conversion efficiency. The tag antenna is an inductively-coupled, meandered dipole that is impedance matched to the tag chip. The dimensions of the tag antenna are 83 x 20mm, and the measured far-field gain of the tag antenna is 1.2 dB. A supply generation subsystem is implemented in order to generate a stable, regulated supply voltage for high-precision sensor interfaces. The supply generation subsystem consists of the low drop-out (LDO) voltage regulator, a bandgap voltage reference, and a supply insensitive current source. The power supply rejection ratio (PSRR) of the LDO is greater than 55 dB. Data communication is realized by backscatter modulation because it offers lower circuit complexity and low power consumption on the tag side. The scattering properties of the tag antenna and the backscatter modulation is theoretically analyzed and an optimum antenna impedance for backscatter communication is calculated. The scattering properties of the tag antenna are characterized by measurements in an anechoic chamber. One of the innovations of this work is the novel pulse-width modulation based backscatter communication, in which the information is encoded in the duty-cycle of the signal. This method decreases the circuit complexity and power consumption on the tag side when used with phase-locked loop (PLL) based sensor interfaces. A monostatic base station with a homodyne receiver is implemented for the passive RFID system. A microstrip patch antenna is custom designed and fabricated for the base station. The input return loss of the base station antenna is measured to be -20 dB, whereas its measured far-field gain is 3.6 dB. An isolator that is based on a directional coupler with an intentional mismatch at the unused port is designed and fabricated for the base station. The measured directivity of the isolator is 50.5 dB. A PLL-based capacitive sensor interface is implemented for differential sensors. The integration of such a low-power circuit in an RFID tag is the main innovation of this work. The differential nature of this sensor interface makes it immune to supply voltage variations. The sensor interface can work with supply voltages as low as 0.3 V. The power consumption of the interface is 300 nW, for 0.3 V supply voltage. The resolution of the sensor interface is determined to be 6.4 bit by post-layout simulations. Two tag architectures for two different approaches for sensor interfaces are proposed: the generic passive UHF RFID platform for sensing applications and the SimpleTag - a novel, low-power UHF RFID tag for differential printed capacitive sensors. The circuits are implemented in UMC 0.18 um CMOS technology.