Two-photon excitation microscopy is a fluorescence imaging technique that allows imaging of living tissue up to about one millimeter in depth. It differs from traditional fluorescence microscopy, in which the excitation wavelength is shorter than the emission wavelength, as the wavelengths of the two exciting photons are longer than the wavelength of the resulting emitted light. Two-photon excitation microscopy typically uses near-infrared excitation light which can also excite fluorescent dyes. However, for each excitation, two photons of infrared light are absorbed. Using infrared light minimizes scattering in the tissue. Due to the multiphoton absorption, the background signal is strongly suppressed. Both effects lead to an increased penetration depth for these microscopes.