Continuous-wave (cw) cavity-ringdown (CRD) spectroscopy provides a highly sensitive way to measure optical absorption by observing the decay rate of light from a high-finesse optical cavity containing the sample of interest (usually gas-phase molecules). In rapidly swept cw-CRD spectroscopy, optical build-up and subsequent ringdown decay are initiated by rapidly sweeping the cavity length or the wavelength of the monochromatic tunable cw laser radiation, thereby establishing and interrupting optical resonance between the laser light and the longitudinal-mode frequencies of the cavity. We review the experimental methodology and applications of this technique, indicating its advantages and prospects for spectroscopic sensing. (C) 2011 Elsevier B. V. All rights reserved.