A method of detecting an analyte is described in which an up-converting luminescent species that is excited by absorption of long wavelength light is provided as a donor of energy to an acceptor species that can be bound to, or generated in proximity to, the luminescent species. The analyte either alters the proximity between the luminescent species and the acceptor species or alters the absorbance or luminescence properties of the acceptor species. The luminescent energy donor is excited with long wavelength light and transfers up-converted energy to the proximate acceptor species and this transfer of energy is detected by measurement of appropriate luminescence properties of either the donor or of the acceptor or both.