Book excerpt: With fast growing consumer demand for high speed mobile data capacity, wireless spectrum has become increasingly precious. This drives the evolution of the personal wireless communication, with new standards developed to improve the spectral efficiency. However, the available spectrum below 10GHz is very limited and packing more bits per second into the same bandwidth requires larger energy consumption as well as more stringent radio and MODEM performance. As a result, such an approach is not sustainable for meeting the future demand. A natural path is to move into higher frequency bands which have larger spectrum bandwidth but less commercial usage. Recent years have witnessed vast technology development on V-band (60GHz) Wireless Personal Area Networks (WPAN) and E-band (80GHz) point-to-point cellular backhauls. Meanwhile, the advancement of low-cost CMOS technologies enables researchers to significantly improve the integration level of high speed mm-wave radios with traditional analog and digital circuitry. However, current mm-wave radio transmitters suffer from short communication distance and low energy efficiency. This is mainly caused by the reduced performance of the CMOS transmitters employing traditional Power Amplifiers (PAs) that suffer from low transistor breakdown voltage, low power gain and poor back-off characteristics. This dissertation investigates the challenges of designing efficient mm-wave transmitters for both long range and short range applications, and proposes concepts and techniques that can potentially break the barriers imposed by the low cost digital CMOS process. The scope of investigation and proposal extends from the architecture level down to the transistor level. Specifically, on-chip and spatial power combining techniques are analyzed and implemented to achieve larger transmitter Equivalent Isotropically Radiated Power (EIRP). To enhance the average efficiency for modulated signals with high Peak-to-Average-Power-Ratio (PAPR), a direct digital-to-RF conversion architecture is proposed and implemented, enabling dynamic DC power scaling. Finally, a Quadrature Spatial Combining concept is introduced to eliminate the tradeoff between low insertion loss and high isolation present in a traditional Cartesian architecture with on-chip signal combiners. Prototype chips are fabricated and tested in 65nm CMOS technology to verify the proposed architectures and techniques.