In the past 10 years extensive effort has been dedicated to commercial wireless local area network (WLAN) systems. Despite all these efforts, however, none of the existing systems has been successful, mainly due to their low data rates. The increasing demand for WLAN systems that can support data rates in excess of 20 Mb/s enticed the FCC to create an unlicensed national information infrastructure (U–NII) band at 5 GHz. This frequency band provides 300 MHz of spectrum in two segments: a 200 MHz(5.15–5.35 GHz) and a 100 MHz (5.725–5.825 GHz) frequency band. This newly released spectrum, and the fast trend of CMOS scaling, provide an opportunity to design WLAN systems with high data rate and low cost. One of the existing standards at 5 GHz is the European high performance radio LAN (HIPERLAN) standard that supports data rates as high as 20 Mb/s. One of the main building blocks of each wireless system is the f- quency synthesizer. Phase–locked loops (PLLs) are universally used to design radio frequency synthesizers. Reducing the power consumption of the frequency dividers of a PLL has always been a challenge. In this book, we introduce an alternative solution for conventional flipflop based xiv MULTI–GHZ FREQUENCY SYNTHESIS & DIVISION frequency dividers. An injection–locked frequency divider (ILFD) takes advantage of the narrowband nature of the wireless systems and employs resonators to trade off bandwidth for power.