Cavity ringdown (CRD) absorption spectroscopy enables spectroscopic sensing of gases with a high sensitivity and accuracy. Instrumental improvements result in a new highperformance continuous-wave (cw) CRD spectrometer using a rapidly-swept cavity of simple design. It employs efficient data-acquisition procedures, high-reflectivity mirrors, a low adsorption flow cell, and various compact fibre-optical components in a single-ended transmitter-receiver configuration suitable for remote sensing. Baseline noise levels in our latest cw-CRD experiments yield a competitive noise-equivalent absorption limit of ~ 5×10⁻¹⁰ cm⁻¹Hz⁻¹/², independent of whatever molecules are to be detected. Measurements in the near-infrared wavelength range of 1.51–1.56 μm yield subppmv (i.e., ppbv or better) sensitivity in the gas phase for several representative molecules (notably CO₂, CO, H₂O, NH₃, C₂H₂, and other hydrocarbons). By measuring spectroscopic features in the 1.525 μm band of C₂H₂ gas, we realise detection limits of 19 nTorr (2.5×10⁻¹¹ atm) of neat C₂H₂ (Doppler-limited at low pressure) and 0.37 ppbv of C₂H₂ in air (pressure-broadened at 1 atm). Our cw-CRD spectrometer is a high-performance sensor in a relatively simple, low-cost, compact instrument that is amenable to chemical analysis of trace gases in medicine, agriculture, industry, and the environment.