The article discusses the advancements in nonlinear laser microscopy, particularly focusing on two- and three-photon excitation (TPE) for biological imaging. The authors highlight the discovery of a wide range of chemical indicators, fluorescent markers, and native biological fluorophores that are suitable for TPE, including NADH, flavins, and green fluorescent proteins. They report measurements of two- and three-photon fluorescence excitation cross sections of various biological indicators, demonstrating the feasibility of using these techniques for deep-tissue imaging. The article also explores the photophysical characteristics of multiphoton excitation, such as the dependence on excitation beam waist size and the impact of intersystem crossing and photobleaching. The authors conclude by discussing the potential applications of three-photon excitation, which offers alternative wavelength windows for probing biological specimens, especially in the deep-UV region. They emphasize the importance of understanding photodamage mechanisms to guide future biological applications.The article discusses the advancements in nonlinear laser microscopy, particularly focusing on two- and three-photon excitation (TPE) for biological imaging. The authors highlight the discovery of a wide range of chemical indicators, fluorescent markers, and native biological fluorophores that are suitable for TPE, including NADH, flavins, and green fluorescent proteins. They report measurements of two- and three-photon fluorescence excitation cross sections of various biological indicators, demonstrating the feasibility of using these techniques for deep-tissue imaging. The article also explores the photophysical characteristics of multiphoton excitation, such as the dependence on excitation beam waist size and the impact of intersystem crossing and photobleaching. The authors conclude by discussing the potential applications of three-photon excitation, which offers alternative wavelength windows for probing biological specimens, especially in the deep-UV region. They emphasize the importance of understanding photodamage mechanisms to guide future biological applications.