The development of a fiber-based, tunable optical cavity with open access is reported. The cavity is of the Fabry-Pérot type and is formed with miniature spherical mirrors positioned on the end of single- or multimode optical fibers by a transfer technique, which involves lifting a high-quality mirror from a smooth convex substrate, either a ball lens or microlens. The cavities typically have a finesse of ~1000 and a mode volume of 600 µm3. The detection of small ensembles of cold Rb atoms guided through such a cavity on an atom chip is demonstrated. © 2006 American Institute of Physics.