An increasing number of researchers are recognizing the distinct advantages offered by molecular fluorescence spectroscopy as an analytical tool. This increase is evident from the large number of manuscripts and monographs published in this area. In the area of fluorescence instrumentation alone, the number of referenced articles in the biennial "Fundamental Reviews" ofAnalytical Chemistryincreased from 82 in 1976 to 156 in 1978. This widespread use of fluorescence analysis arises from its selectivity and sensitivity. The selectivity of fluorescence assay is derived from the dependence of the emission intensity both on the wavelength of the exciting radiation and on the wavelength of detection for the emitted radiation. Thus, it is often possible to find optimal spectral regions where the effects of background or interfering fluorescence are minimized. Moreover, the use of phosphorescence spectra and exploitation of differences in lifetimes and polarization can provide even greater selectivity. The sensitivity advantage of fluorescence arises from the direct detection of the emitted photons to give an emission signal. This is in contrast to absorption spectroscopy, in which a small difference between two large signals is measured. Consequently, with the use of state-of-the-art detectors, the sensitivity of fluorescence methods will often surpass the sensitivity of similar absorption methods by three orders of magnitude.