This lecture covers the principles and applications of two-photon microscopy, focusing on the two-photon absorption process rooted in quantum mechanics. It explains the localization of excitation in deep tissue imaging, comparing it with one-photon excitation. The lecture also discusses the resolution in two-photon microscopy, highlighting its advantages over confocal microscopy. It details the equipment configuration, light source, and image formation in two-photon microscopy. Examples and applications of two-photon fluorescence microscopy are presented, showcasing its advantages in deep tissue imaging and reduced phototoxicity. The lecture concludes by summarizing the benefits of two-photon imaging and highlighting some limitations, such as depth limits and equipment costs.