The emerging field of wireless sensor network (WSN) potentially has a profound impact on our daily life. Widespread deployment of wireless sensor network requires each node to (1) consume less than 100uW of average power for a long usage lifetime and low operational cost, (2) cost less than $1 for a low system cost and (3) occupy less than 1cm3 for seamless integration into our physical environment. Among these requirements, the power constraint is the most challenging. Since communication accounts for majority of power budget in a typical sensor node, it is crucial to have an energy efficient transmitter. In WSN, the radiated power is low (< 1mW) due to the short communication distance (< 10m). As such low radiated power, the overhead power is significant and degrades the transmitter efficiency substantially. This is the reason for the low efficiency of WSN existing transmitters. The thesis focuses on providing a solution to this problem. It first establishes the principles of obtaining an energy efficient transmitter at low radiated power. Based on these principles, three different 1.9GHz transmitters are designed and implemented in ST 0.13um CMOS process: direct modulation transmitter, injection locked transmitter and active antenna transmitter. To push the performance envelope of WSN transmitters, new transmitter architectures, circuit techniques, enabling technologies and co-design methodology are employed. The state-of-the-art active antenna transmitter achieves 46% efficiency and support a data rate up to 330 kbps. Finally, to demonstrate a low power and small form factor sensor node, the active antenna transmitter is integrated into a 38 x 25 x 8.5 mm3 wireless transmit sensor node.