Absorption and luminescence probes are molecules or ions whose spectral properties depend upon the chemical nature of the solubilizing media in the immediate vicinity of the probe molecule. Probe character is generally evidenced by the appearance of new spectral bands, shifts in the absorption or emission wavelengths, or by changes in emission intensities. Probe molecules have been used to determine solvent polarity and solvent acidity/basicity, pH, microviscosity, temperature and/or presence of specific ions/molecules. When combined with fluorescence lifetime measurements, absorption and luminescence probes can provide additional information regarding diffusion coefficients in probe excimer formation, fluidity in lipid vesicles, and rates of intermolecular collisions in quenching experiments.