Chemists have developed tremendous expertise in designing and synthesizing dye molecules which efficiently absorb light of a specified wavelength. Such strongly absorbing molecules have been used for many applications, including dyeing, photography, initiation of chemical reactions, and optical filtering. The probability of a molecule absorbing one photon is proportional to the intensity of the input beam. With sufficiently intense light, such as a laser beam, it is possible for a molecule to absorb simultaneously two photons, each of approximately half the energy (twice the wavelength, λ) normally required to reach an excited state. This two-photon absorption process is intrinsically weak relative to one-photon excitation. The probability of a molecule absorbing two photons simultaneously is proportional to the square of the intensity of the input beam. Advances in recent years have provided a rational basis for the design of efficient two-photon-absorbing molecules and have demonstrated their application in photonics, materials science, and biology.