The ability to observe the fluorescence arising from single molecules has revolutionized our ability to study the structure and dynamics of materials on a microscopic level and to probe the properties of individual members of a heterogeneous ensemble. A variety of near-field and far-field excitation techniques have been employed to study single molecules. Multiphoton excitation (MPE) techniques have a number of advantages that make them particularly attractive for singlemolecule detection. First, because the excitation and fluorescence wavelengths are significantly different from one another, Rayleigh and Raman scattering can easily be filtered out, leading to a low number of background counts. Second, because the probability for MPE depends on the excitation intensity to the second or higher power, the excitation is localized to the point in space where the excitation beam is most tightly focussed, thus providing three-dimensional resolution.