Over the past 30 years, S-band FMCW radars have seen repeated use in atmospheric boundary layer observations. FMCW radars provide tremendous sensitivity and spatial resolution compared to their pulsed counterparts and are therefore attractive for clear-air applications. These instruments are not without limitations, however. Range (height) mislocation errors due to non-zero Doppler velocities and finite coherence of the clear-air echo have implications on both spatial resolution of FMCW radars and on Doppler estimation. Mislocations of O(m) can occur for relatively small velocities using standard signal processing techniques, so some care must be exercised when interpreting echoes from thin interfacial layers. Operating principles of FMCW radar are reviewed and measurement limitations for atmospheric targets are presented. Tradeoff between sensitivity and spatial resolution is discussed. Data collected by the University of Massachusetts' high-resolution S-band FMCW radar is used to illustrate typical performance.