Since the Internet of Things (IoT) is expected to be the new technology to drive the semiconductor industry, significant research efforts have been made to develop new circuit and system techniques for autonomous/very low-power operation of wireless sensor nodes. Very low-power consumption of sensors is key to increase battery lifetime or allow for battery-less (autonomous) operation of sensors, which contributes to preventing or reducing the high maintenance costs of battery supplied sensors and reduce the amount of discarded batteries. This thesis, entitled Radio Frequency Energy Harvesting and Low Power Data Transmission for Autonomous Wireless Sensor Nodes, presents very low-power consumption circuit and system techniques combined with energy harvesting that allow the creation of autonomous wireless sensor nodes. This work focuses on three main challenges: 1) how to improve energy harvesting efficiency, 2) how to minimize power consumption of data transmission and 3) how to combine low-power techniques and energy harvesting in a system. These challenges are addressed in this thesis with on-PCB and Integrated Circuits (IC) solutions. The efficiency of radio frequency (RF) energy harvesting is improved by proposing a new topology of a charge-pump rectifier. The proposed topology uses a voltage boosting network to compensate for the voltage drop in the transistors. The new topology is presented and analyzed. Simulation results are compared to the analytical analysis and measurement results of the circuit that has been fabricated in a 0.18um CMOS technology and operates at 13.53 MHz. Although the efficiency of RF energy harvesting is improved using the above technique, at the same time, low power techniques in data transmission should be developed to save energy. Pulse width modulation and impulse transmission techniques to minimize power consumption have been developed and are presented in this thesis. The developed pulse modulation circuitry has been fabricated in 0.18um CMOS technology as part of a System on Chip (SoC). The new impulse transmitter topology for low-voltage low-power operation has been fabricated on PCB with micro-wave discrete components. Theoretical analysis, simulations and measurements results are shown to prove the impulse transmitter concept. The circuits developed are integrated in a SoC with energy harvesting to prove the concept of autonomous wireless sensor nodes. Two sensor nodes have been designed and measured: one for autonomous temperature monitoring and the second for autonomous ECG monitoring. Both designs operate from wireless power without the use of batteries. Finally, the work developed in this thesis is summarized and future research possibilities are discussed.