When a fluorophore absorbs a photon, an electron is excited to a higher energy level. This excited state electron returns to its ground state by one of two competing processes. In radiative de-excitation, a brief relaxation time (about 10^-12 s) is followed by the electron’s return to the ground state accompanied by the emission of a photon whose wavelength is longer than the one absorbed (1). Fluorescence emission competes with nonradiative processes that also allow the electron to return to the ground state. Because the competition between these processes is influenced by the fluorophore’s surroundings, the nature of the emitted light provides information on the microenvironment of the probe. Therefore, when a fluorophore is part of a macromolecular structure (such as a protein or protein-containing complex), its fluorescence emission provides information on those events occurring within the structure.