The high Q of a microsphere whispering-gallery mode allows for sensitive resonant detection of atoms or molecules. The species being detected absorbs energy from the mode's evanescent field. It can be identified by knowing the resonant wavelength of the driving laser, and its concentration can be determined from the absorption signal on the light in reflection or transmission. High sensitivity results from the long effective absorption path length provided by the whispering-gallery mode's large Q. There are many possible implementations of and applications for such a sensor; several of each are described herein. In particular, for atmospheric trace-gas sensing, the microsphere has the potential to rival the performance of the multipass cell, but in a much more compact and rugged system. Our construction of a prototype system for detection of carbon monoxide, carbon dioxide, and ammonia is described.