The paper investigates the photochemistry of Titan's atmosphere, focusing on the processes that control its composition, climatology, and evolution. The authors use updated chemical schemes and estimates of key rate coefficients to model the atmosphere, incorporating exospheric boundary conditions, vertical transport, and condensation processes at the tropopause. They argue that five major processes govern Titan's atmosphere: photolysis and photosensitized dissociation of CH4, conversion of H to H2 and escape of hydrogen, synthesis of higher hydrocarbons, coupling between nitrogen and hydrocarbons, and coupling between oxygen and hydrocarbons. The model successfully explains the concentrations of minor species observed by Voyager instruments, showing that photochemistry converts simpler atmospheric species into more complex organic compounds, which condense at the tropopause and deposit on the surface. The authors estimate that Titan has lost significant amounts of CH4, N2, and CO over geological time. They also discuss the implications of abiotic organic synthesis on Titan for the origin of life on Earth.The paper investigates the photochemistry of Titan's atmosphere, focusing on the processes that control its composition, climatology, and evolution. The authors use updated chemical schemes and estimates of key rate coefficients to model the atmosphere, incorporating exospheric boundary conditions, vertical transport, and condensation processes at the tropopause. They argue that five major processes govern Titan's atmosphere: photolysis and photosensitized dissociation of CH4, conversion of H to H2 and escape of hydrogen, synthesis of higher hydrocarbons, coupling between nitrogen and hydrocarbons, and coupling between oxygen and hydrocarbons. The model successfully explains the concentrations of minor species observed by Voyager instruments, showing that photochemistry converts simpler atmospheric species into more complex organic compounds, which condense at the tropopause and deposit on the surface. The authors estimate that Titan has lost significant amounts of CH4, N2, and CO over geological time. They also discuss the implications of abiotic organic synthesis on Titan for the origin of life on Earth.