The artificial resonances of dielectric optical microcavities can be used to enhance the detection sensitivity of evanescent-wave optical fluorescence biosensors to the binding of a labeled analyte to a biospecific monolayer. Microcavity sensors offer the high sensitivity of a slab waveguide evanescent fluorescence sensor of much larger sensing surface area. Alternatively, when compared to a slab waveguide of the same area, microcavity based sensors offer much improved sensitivity. In either case, the required number of bound analytes is dramatically reduced. These scaling relations are highly conducive to achieving large sensing arrays with small sample volumes.