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In TPA, the absorption is directly proportional to the square of incident light intensity, and thus lasers are required for excitation. The advantages of TPA microscopy include better focus and less out-of-focus bleaching, together with absorption at longer wavelengths than in one-photon absorption, which leads to deeper penetration in scattering media, such as tissues. However, TPA probes are usually associated with less sensitivity, and thus designing TPA fluorophores with large absorption probability is an important area of research. TPA of biological molecules like fluorescent proteins and nucleic acids is of particular interest. These molecules are experimentally produced through utilization of the naturally present transcription mechanism in the cell and thus pose less cell toxicity. In this chapter, we review the theory of TPA highlighting the computational approaches used to study biological molecules. We discuss the computational methods available for exploring TPA and recent computational studies on the TPA of fluorescent proteins and nucleic acid base analogues. The chapter concludes by highlighting possible research avenues and unanswered questions.enAbsorptionBiological MoleculesArticle